Semiconductor device

ABSTRACT

[Problem] To provide a semiconductor device suitable for miniaturization. To provide a highly reliable semiconductor device. To provide a semiconductor device with improved operating speed. 
     [Solving Means] A semiconductor device including a memory cell including first to cth (c is a natural number of 2 or more) sub memory cells, wherein: the jth sub memory cell includes a first transistor, a second transistor, and a capacitor; a first semiconductor layer included in the first transistor and a second semiconductor layer included in the second transistor include an oxide semiconductor; one of terminals of the capacitor is electrically connected to a gate electrode included in the second transistor; the gate electrode included in the second transistor is electrically connected to one of a source electrode and a drain electrode which are included in the first transistor; and when j≧2, the jth sub memory cell is arranged over the j−1th sub memory cell.

TECHNICAL FIELD

The present invention relates to an object, a method, or a manufacturingmethod. Furthermore, the present invention relates to a process, amachine, manufacture, or a composition of matter (composition ofmatter). In particular, one embodiment of the present invention relatesto a semiconductor device, a display device, a light-emitting device, apower storage device, a memory device, a driving method thereof, or amanufacturing method thereof.

Note that in this specification and the like, a semiconductor devicerefers to all devices that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. Furthermore, an arithmetic device,a memory device, an imaging device, an electro-optical device, a powergeneration device (including a thin film solar cell, an organic thinfilm solar cell, and the like), and an electronic appliance each mayinclude a semiconductor device.

BACKGROUND ART

A technique in which a transistor is formed using a semiconductormaterial has attracted attention. The transistor is applied to a widerange of electronic devices such as an integrated circuit (IC) or animage display device (also simply referred to as a display device). Assemiconductor materials applicable to the transistor, silicon-basedsemiconductor materials have been widely known, but oxide semiconductorshave been attracting attention as alternative materials.

For example, a technique for forming a transistor using zinc oxide or anIn—Ga—Zn-based oxide semiconductor as an oxide semiconductor isdisclosed (see Patent Literature 1 and Patent Literature 2).

Furthermore, in recent years, demand for integrated circuits in whichsemiconductor elements such as miniaturized transistors are integratedwith high density has risen with increased performance and reductions inthe size and weight of electronic appliances.

CITATION LIST Patent Literatures

[Patent Literature 1] Japanese Published Patent Application No.2007-123861

[Patent Literature 2] Japanese Published Patent Application No.2007-096055

SUMMARY OF INVENTION Problems to be Solved by the Invention

One object of one embodiment of the present invention is to provide asemiconductor device that is suitable for miniaturization. Anotherobject is to provide a semiconductor device having a reduced circuitarea. Another object of one embodiment of the present invention is toprovide a semiconductor device with improved operating speed. Anotherobject of one embodiment of the present invention is to provide asemiconductor device with improved write speed. Another object of oneembodiment of the present invention is to provide a semiconductor devicewith improved read speed. Another object of one embodiment of thepresent invention is to provide a semiconductor device with low powerconsumption.

Another object of one embodiment of the present invention is to providea highly reliable semiconductor device. Another object of one embodimentof the present invention is to give favorable electrical characteristicsto a semiconductor device. Another object of one embodiment of thepresent invention is to provide a semiconductor device including amemory element with favorable retention characteristics. Another objectof one embodiment of the present invention is to provide a semiconductordevice having a novel structure. Another object is to provide a novelsemiconductor device.

Note that the descriptions of these objects do not disturb the existenceof other objects. Note that in one embodiment of the present invention,there is no need to achieve all the objects. Note that other objectswill be apparent from the description of the specification, thedrawings, the claims, and the like and can be derived from thedescription of the specification, the drawings, the claims, and thelike.

Means for Solving the Problems

One embodiment of the present invention is a semiconductor deviceincluding a memory cell including first to cth (c is a natural number of2 or more) sub memory cells, wherein: the jth (j is a natural number of1 to c) sub memory cell includes a first transistor, a secondtransistor, and a capacitor; a first semiconductor layer included in thefirst transistor and a second semiconductor layer included in the secondtransistor include an oxide semiconductor; one of terminals of thecapacitor is electrically connected to a gate electrode included in thesecond transistor; the gate electrode included in the second transistoris electrically connected to one of a source electrode and a drainelectrode which are included in the first transistor; and when j≧2, thejth sub memory cell is arranged over a j−1th sub memory.

Furthermore, one embodiment of the present invention is a semiconductordevice including a memory cell including first to cth (c is a naturalnumber of 2 or more) sub memory cells, wherein: the jth (j is a naturalnumber of 1 to c) sub memory cell includes a first transistor, a secondtransistor, and a capacitor; a first semiconductor layer included in thefirst transistor and a second semiconductor layer included in the secondtransistor include an oxide semiconductor; one of terminals of thecapacitor is electrically connected to a gate electrode included in thesecond transistor; the gate electrode included in the second transistoris electrically connected to one of a source electrode and a drainelectrode which are included in the first transistor; when j≧2, thesemiconductor layer included in the second transistor Tb_j included inthe jth sub memory cell and a semiconductor layer included in a firsttransistor Ta_(j−1) included in the j−1th sub memory cell are in contactwith an upper surface of a first insulating film; and when j≧2, the gateelectrode included in the second transistor Tb_j included in the jth submemory cell and a gate electrode included in the first transistorTa_(j−1) included in the j−1th sub memory cell are in contact with alower surface of a second insulating film.

Furthermore, one embodiment of the present invention is a semiconductordevice including a memory cell including first to cth (c is a naturalnumber of 2 or more) sub memory cells, wherein: the jth (j is a naturalnumber of 1 to c) sub memory cell includes a first transistor, a secondtransistor, and a capacitor; when j≧2, the jth sub memory cell isarranged over a j−1th sub memory; a first semiconductor layer includedin the first transistor and a second semiconductor layer included in thesecond transistor include an oxide semiconductor; one of thesemiconductor layer included in the first transistor and thesemiconductor layer included in the second transistor which are includedin the first sub memory cell, and a semiconductor layer included in athird transistor are over and in contact with a first insulating film;and one of a semiconductor layer included in a first transistor includedin the cth sub memory cell and a semiconductor layer included in asecond transistor included in the cth sub memory cell, and asemiconductor layer included in a fourth transistor are over and incontact with a second insulating film.

Furthermore, in the above structure, it is preferable that the firstsemiconductor layer included in the first transistor included in the jth(j is a natural number of 1 to c) sub memory cell and the secondsemiconductor layer included in the second transistor included in thejth sub memory cell include In, an element represented by M, and Zn,that the atomic ratio of In to M and Zn of the oxide semiconductorincluded in the first semiconductor layer satisfy In:M:Zn=g:h:i, thatthe atomic ratio of In to M and Zn of the oxide semiconductor includedin the second semiconductor layer satisfy In:M:Zn=d:e:f, and thatg/(g+h+i) be smaller than d/(d+e+f).

Advantageous Effects of the Invention

One embodiment of the present invention can provide a semiconductordevice that is suitable for miniaturization. Furthermore, asemiconductor device having a reduced circuit area can be provided.Furthermore, one embodiment of the present invention can provide asemiconductor device with improved operating speed. Furthermore, oneembodiment of the present invention can provide a semiconductor devicewith improved write speed. Furthermore, one embodiment of the presentinvention can provide a semiconductor device with improved read speed.Furthermore, one embodiment of the present invention can provide asemiconductor device with low power consumption.

Furthermore, one embodiment of the present invention can provide ahighly reliable semiconductor device. Furthermore, one embodiment of thepresent invention can provide a semiconductor device with favorableelectrical characteristics. Furthermore, one embodiment of the presentinvention can provide a semiconductor device including a memory elementwith favorable retention characteristics. Furthermore, one embodiment ofthe present invention can provide a semiconductor device having a novelstructure. Furthermore, a novel semiconductor device can be provided.

Note that the descriptions of these effects do not disturb the existenceof other effects. Note that in one embodiment of the present invention,there is no need to achieve all the effects. Note that other effectswill be apparent from the description of the specification, thedrawings, the claims, and the like and can be derived from thedescription of the specification, the drawings, the claims, and thelike.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B Drawings showing an example and a circuit diagram of asemiconductor device of one embodiment of the present invention.

FIG. 2 A block diagram of one embodiment of the present invention.

FIGS. 3A and 3B Drawings illustrating timing charts of a circuit of oneembodiment of the present invention.

FIG. 4 A circuit diagram of one embodiment of the present invention.

FIG. 5 A circuit diagram of one embodiment of the present invention.

FIG. 6 A drawing showing an example of a semiconductor device of oneembodiment of the present invention.

FIGS. 7A to 7E Drawings showing examples of a semiconductor device ofone embodiment of the present invention.

FIG. 8 A drawing showing an example of a semiconductor device of oneembodiment of the present invention.

FIG. 9 A drawing showing an example of a semiconductor device of oneembodiment of the present invention.

FIG. 10 A drawing showing an example of a semiconductor device of oneembodiment of the present invention.

FIGS. 11A to 11C Drawings showing an example of a semiconductor deviceof one embodiment of the present invention.

FIG. 12 A drawing showing an example of a semiconductor device of oneembodiment of the present invention.

FIGS. 13A to 13E Drawings showing a method for manufacturing asemiconductor device of one embodiment of the present invention.

FIGS. 14A to 14C Drawings showing the method for manufacturing asemiconductor device of one embodiment of the present invention.

FIGS. 15A to 15C Drawings showing the method for manufacturing asemiconductor device of one embodiment of the present invention.

FIGS. 16A and 16B Drawings showing the method for manufacturing asemiconductor device of one embodiment of the present invention.

FIGS. 17A and 17B Drawings showing an example of a transistor.

FIGS. 18A to 18E Drawings showing examples of a semiconductor device ofone embodiment of the present invention.

FIGS. 19A to 19E Drawings showing examples of a semiconductor device ofone embodiment of the present invention.

FIGS. 20A to 20D Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of theCAAC-OS.

FIGS. 21A to 21D Cs-corrected high-resolution TEM images of a plane of aCAAC-OS.

FIGS. 22A to 22C Drawings illustrating structural analysis of a CAAC-OSand a single crystal oxide semiconductor by XRD.

FIGS. 23A and 23B Drawings showing electron diffraction patterns of aCAAC-OS.

FIG. 24 A drawing showing a change of crystal parts of an In—Ga—Zn oxideowing to electron irradiation.

FIGS. 25A and 25B A drawing showing the band structure of part of atransistor of one embodiment of the present invention and a drawingillustrating a current path when the transistor is on.

FIGS. 26A and 26B Circuit diagrams of an embodiment.

FIG. 27 A configuration example of an RF tag of an embodiment.

FIG. 28 A configuration example of a CPU of an embodiment.

FIG. 29 A circuit diagram of a memory element of an embodiment.

FIGS. 30A to 30F Electronic appliances of an embodiment.

FIGS. 31A to 31F Application examples of an RF tag of an embodiment.

MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail with reference to drawings. Notethat the present invention is not limited to the description below, andit is easily understood by those skilled in the art that the modes anddetails thereof can be variously changed without departing from thepurpose and the scope of the present invention. Accordingly, the presentinvention should not be interpreted as being limited to the content ofthe embodiments below.

Note that in the structures of the invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description of suchportions is not repeated. Further, the same hatching pattern is appliedto portions having similar functions, and the portions are notespecially denoted by reference numerals in some cases.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases; therefore, it is not necessarily limited to thescales.

Note that in this specification and the like, ordinal numbers such as“first”, “second”, and the like are used in order to avoid confusionamong components and do not limit the number.

Note that a “semiconductor” includes characteristics of an “insulator”in some cases when the conductivity is sufficiently low, for example.Further, a “semiconductor” and an “insulator” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “insulator” is not clear. Accordingly, a“semiconductor” in this specification can be called an “insulator” insome cases. Similarly, an “insulator” in this specification can becalled a “semiconductor” in some cases.

Further, a “semiconductor” includes characteristics of a “conductor” insome cases when the conductivity is sufficiently high, for example.Further, a “semiconductor” and a “conductor” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “insulator” is not clear. Accordingly, a“semiconductor” in this specification can be called a “conductor” insome cases. Similarly, a “conductor” in this specification can be calleda “semiconductor” in some cases.

A transistor is a kind of semiconductor elements and can achieveamplification of current or voltage, switching operation for controllingconduction or non-conduction, or the like. A transistor in thisspecification includes an IGFET (Insulated Gate Field Effect Transistor)and a thin film transistor (TFT: Thin Film Transistor).

Furthermore, in this specification, the term “parallel” indicates astate in which the angle formed between two straight lines is greaterthan or equal to −10° and less than or equal to 10°, and accordinglyalso includes the case where the angle is greater than or equal to −5°and less than or equal to 5°. Furthermore, the term “substantiallyparallel” indicates a state in which the angle formed between twostraight lines is greater than or equal to −30° and less than or equalto 30°. In addition, the term “perpendicular” indicates a state in whichthe angle formed between two straight lines is greater than or equal to80° and less than or equal to 100°, and accordingly also includes thecase where the angle is greater than or equal to 85° and less than orequal to 95°. Furthermore, the term “substantially perpendicular”indicates a state in which the angle formed between two straight linesis greater than or equal to 60° and less than or equal to 120°.

Furthermore, in this specification, trigonal and rhombohedral crystalsystems are included in a hexagonal crystal system.

Embodiment 1

In this embodiment, a circuit configuration and operation of a memorycell array 300 included in a semiconductor device 700 of one embodimentof the invention to be disclosed are described.

Note that a semiconductor device refers to a device including asemiconductor element. Note that a semiconductor device includes adriver circuit or the like for driving a circuit including asemiconductor element. Furthermore, in some cases, a semiconductordevice includes a driver circuit, a power supply circuit, or the likeprovided over another substrate, in addition to a memory cell.

Furthermore, an inverter circuit, a NAND circuit, an AND circuit, a NORcircuit, an OR circuit, a buffer, a level shifter, an XOR circuit, anXNOR circuit, an AND-NOR circuit, an OR-NAND circuit, an AND-OR-NVcircuit, an OR-AND-INV circuit, an analog switch, a flip-flop, asettable flip-flop, a resettable flip-flop, a settable and resettableflip-flop, an adder, a half adder, a multiplexer, a demultiplexer, aregister, a scan register, a retention register, an isolator, a decoder,or the like may be included in the semiconductor device 700.

An example of the semiconductor device 700 of one embodiment of thepresent invention is shown in FIG. 2. The semiconductor device 700includes the memory cell array 300 and a peripheral circuit 500 of thememory cell array. Furthermore, the peripheral circuit 500 of the memorycell array preferably includes a row selection driver, a columnselection driver, an A/D converter, and the like. Furthermore, theperipheral circuit 500 may include a logic circuit or the like.Furthermore, a structure of the semiconductor device 700 is not limitedto the structure shown in FIG. 2.

Here, the structure including the memory cell array 300 and the rowselection driver, the column selection driver, the A/D converter, andthe like that are connected to the memory cell array may be referred toas a memory device.

The memory cell array 300 shown in FIG. 1A includes memory cells CLarranged in a matrix of a (in the horizontal direction)×b (in thevertical direction) (a and b are natural numbers) in a plane.

Each memory cell CL includes c (c is a natural number of 2 or more) submemory cells SCL. Here, a jth (j is a natural number of 1 to c) submemory cell is denoted by SCL_j. The sub memory cell SCL_j includes afirst transistor Ta_j, a second transistor Tb_j, and a first capacitorCa_j.

That is, the memory cell CL includes the c sub memory cells SCL_j whichare stacked, and each sub memory cell SCL includes a first transistorTa, a second transistor Tb, and a capacitor Ca.

For example, the case where single crystal silicon is used for thetransistor Ta_j and the transistor Tb_j is considered. In order toobtain excellent single crystal silicon, a transistor is preferablyformed using a single crystal silicon substrate or the like. Meanwhile,in the case where oxide semiconductor layers are used for the transistorTa_j and the transistor Tb_j, because they can be formed by, forexample, a sputtering method which is described later, a CVD method, anMBE method, a PLD method, an ALD method, or the like, semiconductorlayers can be formed by being stacked repeatedly. Therefore, atransistor can be formed by being stacked over a transistor. As shown inFIG. 1A, c sub memory cells SCL can be stacked. The memory cell CLincludes the c sub memory cells SCL. Accordingly, capacity per area canbe increased.

As shown in FIG. 1B, in the memory cell CL including the c sub memorycells SCL, the sub memory cells SCL_j share a bit line BL and a sourceline SL.

A write word line WWL_j is connected to a gate of the transistor Ta_j.Furthermore, the bit line BL is connected to one of a source and a drainof the transistor Ta_j, and a floating node FN is connected to the otherof the source and the drain thereof.

The floating node FN is connected to a gate of the transistor Tb_j.Furthermore, the bit line BL is connected to one of a source and a drainof the transistor Tb_j, and the source line SL is connected to the otherof the source and the drain thereof.

The floating node FN is connected to one electrode of the capacitorCa_j, and a read word line RWL_j is connected to the other electrodethereof.

A word signal is supplied to the write word line WWL_j.

The word signal is a signal to turn on the transistor Ta_j to supply thepotential of the bit line BL to the floating node FN.

Binary or multilevel data is supplied to the bit line BL. The multileveldata is k-bit (k is a natural number of 2 or more) data. Specifically,2-bit data is 4-level data, namely, a signal having any one of the fourlevels of voltages.

A read signal is supplied to the read word line RWL_j.

The read signal is a signal which is supplied to the other electrode ofthe capacitor Ca_j to perform reading of data from the memory cell in aselective manner.

The floating node FN corresponds to any node on a wiring which connectsthe one electrode of the capacitor Ca_j, the other electrode of thesource and the drain of the transistor Ta_j, and the gate of thetransistor Tb_j.

Note that in this specification, node refers to any point on a wiringprovided to connect elements electrically.

Note that in this specification, writing of data to the memory cellmeans that a word signal supplied to the write word line WWL_j iscontrolled so that the potential of the floating node FN becomes apotential corresponding to the potential of the bit line BL.Furthermore, reading of data from the memory cell means that a readsignal supplied to the read word line RWL_j is controlled so that thepotential of the bit line BL becomes a potential corresponding to thepotential of the floating node FN.

The transistor Ta_j preferably has a second gate electrode (BG). Apotential lower or higher than that of the source electrode can beapplied to the second gate electrode, whereby the threshold voltage ofthe transistor can be shifted in a positive direction or a negativedirection. For example, by shifting the threshold voltage of thetransistor in the positive direction, normally-off in which thetransistor is in a non-conduction state (off state) even when the gatepotential is 0 V can be achieved in some cases. Note that the voltageapplied to the second gate electrode may be variable or fixed. In thecase where the voltage applied to the second gate electrode is variable,a circuit for controlling the voltage may be connected to the secondgate electrode. Furthermore, the second gate electrode may be connectedto a first gate electrode. The second gate is connected to the firstgate and the same potential is applied thereto, whereby on-state currentcan be increased, variations in the initial characteristics can bereduced, degradation due to the −GBT (Minus Gate Bias Temperature)stress test can be suppressed, and a change in the rising voltage of theon-state current at different drain voltages can be suppressed.

In addition, although not illustrated in FIG. 1B, the transistor Tb_jmay also have a second gate electrode (BG). The on-state current of thetransistor Tb_j is preferably high. The increase of the on-state currentof the transistor Tb_j can increase the read speed of the memory cellarray 300, for example.

Note that in the case of including a display element such as a liquidcrystal element or an organic EL (Electroluminescence) elementelectrically connected to the node FN, for example, a part of the memorycell array 300 can function as a pixel of a display device.

Note that the potential of the floating node FN is based on the datasupplied to the bit line BL. Furthermore, the floating node FN is in anelectrically floating state when the transistor Ta_j is in anon-conduction state. Thus, in the case where the voltage of the readsignal supplied to the read word line RWL is changed, the potential ofthe floating node FN becomes a potential which is the sum of theprevious potential and the amount of change in the voltage of the readsignal. The change in the potential is due to capacitive coupling of thecapacitor Ca_j which is caused by the change of the read signal suppliedto the read word line RWL.

The transistor Ta_j has a function of a switch for controlling writingof data by being switched between a conduction state and anon-conduction state. Furthermore, the transistor Ta_j has a function ofretaining a potential based on written data by keeping a non-conductionstate. Note that the transistor Ta_j is also referred to as a firsttransistor. Furthermore, the transistor Ta_j is an n-channel typetransistor in the description.

Note that, as the transistor Ta_j, a transistor having a low current(off-state current) which flows between a source and a drain in anon-conduction state is preferably used. Here, the low off-state currentmeans that the normalized off-state current per micrometer of a channelwidth with a drain-source voltage of 10 V at room temperature is lessthan or equal to 10 zA. An example of a transistor having such a lowoff-state current is a transistor including an oxide semiconductor as asemiconductor layer.

A transistor with low off-state current is used as the transistor Ta_j,whereby the potential of the floating node FN in a non-conduction statecan be retained for a long period of time. Thus, the refresh rate of thesemiconductor device can be reduced, which achieves alow-power-consumption semiconductor device.

Note that to hold a potential retained in the floating node FN at 85° C.for 10 years (3.15×10⁸ seconds), a value of off-state current ispreferably lower than 4.3 yA (yoctoamperes: 1 yA is 10⁻²⁴ A) perfemtofarad of capacitance and per micrometer of a channel width of thetransistor. In that case, the allowable potential variation in thefloating node FN is preferably within 0.5 V. Alternatively, theoff-state current is preferably lower than 1.5 yA at 95° C. In thesemiconductor device of one embodiment of the present invention, theconcentration of hydrogen contained in the layer below the barrier filmis sufficiently reduced. Thus, the transistor including an oxidesemiconductor in the layer over the barrier film can have extremely lowoff-state current.

Furthermore, when the capacitance is increased, the potential can beretained in the node FN for a longer time. In other words, the retentiontime can be lengthened.

In the configuration of the memory cell array 300 shown in FIG. 1B, apotential based on written data is retained by keeping thenon-conduction state. Thus, it is particularly preferable to use atransistor with a low off-state current as a switch for suppressingchange in the potential in the floating node FN which is accompanied bythe transfer of electrical charge.

When the transistor Ta_j is a transistor having a low off-state currentand keeps a non-conduction state, the memory cell array 300 can be anon-volatile memory. Thus, once data is written to the memory cell array300, the data can be retained in the floating node FN until thetransistor Ta_j is turned on again.

The transistor Tb_j has a function of making a current Id flow betweenthe source and the drain in accordance with the potential of thefloating node FN. Note that in the memory cell array 300 shown in FIG.1A, the current Id that flows between the source and the drain of thetransistor Tb_j is a current that flows between the bit line BL and thesource line SL. Note that as the transistor Tb_j, a transistor usingsilicon in a semiconductor layer may be used, or a transistor using anoxide semiconductor in a semiconductor layer may be used. Here, anexample in which a transistor using an oxide semiconductor in asemiconductor layer is used as the transistor Tb_j is shown. Note thatthe transistor Tb_j is also referred to as a second transistor.Furthermore, the transistor Tb_j is an n-channel type transistor in thedescription.

An n-channel type transistor which has high switching speed can be usedfor the transistor Ta_j and the transistor Tb_j. For example, theswitching speed of the transistor is lower than 10 ns, preferably lowerthan 1 ns, more preferably lower than 0.1 ns. For example, a transistorincluding an oxide semiconductor (preferably an oxide including In, Ga,and Zn) in a channel formation region (hereinafter the transistor isalso referred to as a transistor using an oxide semiconductor) can beused.

Next, operation of the memory cell array 300 illustrated in FIG. 1B isdescribed.

Timing charts shown in FIGS. 3A and 3B illustrate change of signalssupplied to the write word line WWL, the read word line RWL, thefloating node FN, the bit line BL, and the source line SL which areshown in FIG. 1B.

First, write operation is described with reference to FIG. 3A. Thoughwriting of binary data is described here, writing to the memory cellarray 300 is not limited to the writing of binary data, and multileveldata may be written. In the timing chart shown in FIG. 3A, a writingperiod T4, a break period T5, and a non-selection period T6 are shown.

In the writing period T4, a potential V2 is supplied to the write wordline WWL first. Furthermore, a potential V0 is supplied to the read wordline RWL. Next, a potential corresponding to binary data, that is, anH-level potential or an L-level potential is supplied to the bit lineBL. Furthermore, the H-level potential is supplied to the source lineSL.

Next, in the break period T5, the potential V0 is supplied to the readword line RWL and the write word line WWL. Next, the L-level potentialis supplied to the bit line BL and the source line SL. Here, thepotential V0 may be a ground potential, and the potential V2 may be apositive potential. Furthermore, the absolute value of the potential V2is preferably higher than the H-level potential. For example, it mayrange from approximately a threshold value of the transistor Tb_j toapproximately three times the threshold value.

Next, in the non-selection period T6, a potential V1 is applied to theread word line RWL and the write word line WWL. Here, the potential V1may be a negative potential, for example. Furthermore, the absolutevalue of the potential V1 is preferably higher than the H-levelpotential. Furthermore, the L-level potential is supplied to the bitline BL and the source line SL.

Next, read operation is described with reference to FIG. 3B. In thetiming chart shown in FIG. 3B, a period T1 in which the potential of thebit line BL is precharged, a period T2 in which the electrical charge ofthe bit line BL is discharged to perform data reading, and anon-selection period T3 are shown.

In the period T1 shown in FIG. 3B, the bit line BL is precharged. Thatis, the bit line BL is supplied with a potential (a potential H′) whichis almost the same as that of the H-level. At this time, the potentialV1 is supplied to the write word line WWL. Furthermore, the read wordline RWL is supplied with the potential V1. Furthermore, in the floatingnode FN, a potential corresponding to data is retained. Furthermore, thesource line SL is supplied with the L-level potential.

At this time, the bit line BL becomes in an electrically floating stateafter being supplied with the H-level potential. That is, the bit lineBL is brought into a state in which the potential is changed by thecharging or discharging of electrical charge. The floating state can beachieved by turning off a switch for supplying a potential to the bitline BL.

Next, in the period T2 shown in FIG. 3B, the electrical charge of thebit line BL is discharged to perform data reading. At this time, thewrite word line WWL is supplied with the potential V1 as in the previousperiod. Furthermore, the read word line RWL is supplied with the H-levelpotential, here, the potential V0. Furthermore, in the floating node FN,each of the potentials corresponding to data is increased. Furthermore,the potential of the bit line BL is changed in accordance with thepotential of the floating node FN. For example, an H-level signal (thepotential H′) is output to the bit line BL in the case where the L-levelpotential is input to the floating node FN, and an L-level signal (apotential L′) is output to the bit line BL in the case where the H-levelpotential is input to the floating node FN. Furthermore, the L-levelpotential is supplied to the source line SL as in the previous period.

Next, the period T3 shown in FIG. 3B shows a non-selection state. In theperiod T3, a potential of the read word line RWL is set to V1.

Furthermore, the memory cell array 300 may have a circuit configurationas shown in FIG. 4. In FIG. 4, the sub memory cells SCL_j are connectedto the bit line BL in a staggered configuration, whereby the integrationdegree of the memory cell array 300 can be increased in some cases.Furthermore, the storage capacity per area can be increased. Here, amongthe sub memory cells SCL_j where j=1 to c, four sub memory cells SCL_jwhere j=1, 2, 3, and c are shown in FIG. 4. Furthermore, the memory cellarray 300 may have a circuit configuration as shown in FIG. 5. In FIG.5, the number of source lines SL can be smaller than that in FIG. 4.Here, among the sub memory cells SCL_j where j=1 to c, four sub memorycells SCL_j where j=1, 2, 3, and c are shown in FIG. 5. Note that alsoin FIG. 4 and FIG. 5, the transistor Ta_j may have a second gateelectrode (BG) in a manner similar to that of FIG. 1B.

Structure Example of Stacked-layer Structure

Next, an example of a stacked-layer structure including the memory cellarray 300 illustrated in FIGS. 1A and 1B is described using FIG. 6.

A stacked-layer structure shown in FIG. 6 is an example of thesemiconductor device 700 including the memory cell array 300 and theperipheral circuit 500.

The peripheral circuit 500 includes a transistor 130 a, a transistor 130b, a transistor 230 a, and a transistor 230 b. The transistor 130 a, thetransistor 130 b, the transistor 230 a, and the transistor 230 b containa first semiconductor material. As semiconductors that can be used asthe first semiconductor material, semiconductor materials such assilicon, germanium, gallium, and arsenic; compound semiconductormaterials containing silicon, germanium, gallium, arsenic, or aluminum;organic semiconductor materials; oxide semiconductor materials; or thelike is given. Here, the case where single crystal silicon is used asthe first semiconductor material is described. Although the transistor130 a, the transistor 130 b, the transistor 230 a, and the transistor230 b may be either a p-channel type or an n-channel type, anappropriate transistor may be used depending on the circuitconfiguration or the driving method. Here, n-channel type transistorsare given as examples of the transistor 130 a and the transistor 130 b,and p-channel type transistors are given as examples of the transistor230 a and the transistor 230 b. Here, because the transistor 130 a andthe transistor 130 b have almost similar structures, only the transistor130 a is described. Furthermore, because the transistor 230 a and thetransistor 230 b are almost similar in structure, only the transistor230 a is described.

The transistor 130 a is provided for a semiconductor substrate 131 andincludes a semiconductor layer 132 which is part of the semiconductorsubstrate 131, a gate insulating film 134, a gate electrode 135, and alow-resistance layer 133 a and a low-resistance layer 133 b eachfunctioning as a source region or a drain region.

A region of the semiconductor layer 132 where a channel is formed, aregion in the vicinity thereof, the low-resistance layer 133 a and thelow-resistance layer 133 b to be a source region or a drain region, andthe like preferably contain a semiconductor such as a silicon-basedsemiconductor, more preferably contain single crystal silicon.Alternatively, it may be formed of a material including Ge (germanium),SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (galliumaluminum arsenide), or the like. Alternatively, a structure usingsilicon having crystal lattice distortion may be employed.Alternatively, the transistor 130 a may be a HEMT (High ElectronMobility Transistor) with GaAs, GaAlAs, or the like.

Furthermore, the transistor 130 a may include a region 176 a and aregion 176 b which are LDD (lightly doped drain) regions.

The low-resistance layer 133 a and the low-resistance layer 133 bcontain an element that imparts n-type conductivity, such as phosphorus,or an element that imparts p-type conductivity, such as boron, inaddition to a semiconductor material used for the semiconductor layer132.

For the gate electrode 135, a semiconductor material such as siliconcontaining the element that imparts n-type conductivity, such asphosphorus, or the element that imparts p-type conductivity, such asboron, or a conductive material such as a metal material, an alloymaterial, or a metal oxide material can be used. It is particularlypreferable to use a high-melting-point material that has both heatresistance and conductivity, such as tungsten or molybdenum, and it isparticularly preferable to use tungsten.

The transistor 230 a is provided for the semiconductor substrate 131 andincludes a semiconductor layer 232 which is part of the semiconductorsubstrate 131, the gate insulating film 134, a gate electrode 235, and alow-resistance layer 233 a and a low-resistance layer 233 b eachfunctioning as a source region or a drain region.

For the semiconductor layer 232, the description of the semiconductorlayer 132 may be referred to. Furthermore, for the low-resistance layer233 a and the low-resistance layer 233 b, the description of thelow-resistance layer 133 a and the low-resistance layer 133 b may bereferred to. Furthermore, for the gate electrode 235, the description ofthe gate electrode 135 may be referred to.

Furthermore, for example, in the case where the transistor 130 a is ann-channel type transistor and the transistor 230 a is a p-channel typetransistor, for example, phosphorus may be added to the low-resistancelayer 133 a and the low-resistance layer 133 b, and, for example, boronmay be added to the low-resistance layer 233 a and the low-resistancelayer 233 b. Furthermore, for example, materials with different workfunctions may be used for the gate electrode 135 and the gate electrode235.

Here, a transistor 190 as shown in FIGS. 17A and 17B may be used insteadof the transistor 130 a, the transistor 130 b, the transistor 230 a, andthe transistor 230 b. Note that an example of an n-channel typetransistor is shown in FIGS. 17A and 17B, but a similar structure can beused for a p-channel type transistor. A cross section along dasheddotted line A-B illustrated in FIG. 17A is shown in FIG. 17B. In thetransistor 190, the semiconductor layer 132 (part of the semiconductorsubstrate 131) in which a channel is formed has a protruding shape, andthe gate insulating film 134 and the gate electrode 135 are providedalong a side surface and an upper surface of the protruding portion. Thetransistor 190 is also referred to as a FIN transistor because itutilizes a protruding portion of the semiconductor substrate. Note thatan insulating film serving as a mask for forming the protruding portionmay be provided in contact with the top of the protruding portion.Furthermore, although the case where the protruding portion is formed byprocessing part of the semiconductor substrate is described here, asemiconductor layer having a protruding shape may be formed byprocessing an SOI substrate.

An insulating film 136, an insulating film 137, and an insulating film138 are sequentially stacked to cover the transistor 130 a, thetransistor 130 b, the transistor 230 a, and the transistor 230 b.

In a manufacturing process of the semiconductor device, the insulatingfilm 136 functions as a protective film at the time of activating anelement imparting conductivity that is added to the low-resistance layer133 a, the low-resistance layer 133 b, the low-resistance layer 233 a,the low-resistance layer 233 b, and the like. The insulating film 136 isnot necessarily provided when not needed.

In the case where a silicon-based semiconductor material is used for thesemiconductor layer 132 and the semiconductor layer 232, the insulatingfilm 137 preferably includes an insulating material containing hydrogen.The insulating film 137 containing hydrogen is provided over thetransistor 130 a, transistor 130 b, the transistor 230 a, and thetransistor 230 b, and heat treatment is performed, whereby danglingbonds in the semiconductor layer 132 and the semiconductor layer 232 areterminated by hydrogen in the insulating film 137, so that thereliability of the transistor 130 a, transistor 130 b, the transistor230 a, and the transistor 230 b can be improved.

The insulating film 138 functions as a planarization layer thatplanarizes a level difference caused by the transistor 130 a, thetransistor 130 b, the transistor 230 a, the transistor 230 b, and thelike that are provided in a layer thereunder. The upper surface of theinsulating film 138 may be planarized by planarization treatment using aCMP (Chemical Mechanical Polishing) method or the like in order toincrease the planarity of the upper surface thereof.

Furthermore, a plug or the like that is electrically connected to thelow-resistance layer 133 a, the low-resistance layer 133 b, thelow-resistance layer 233 a, the low-resistance layer 233 b, and the likemay be embedded in the insulating film 136, the insulating film 137, andthe insulating film 138.

Furthermore, a transistor using an oxide semiconductor is included inthe memory cell array 300. Therefore, the stacked-layer structure shownin FIG. 6 preferably includes a barrier film 111 between the memory cellarray 300 and the transistor 130 a, the transistor 130 b, the transistor230 a, and the transistor 230 b.

The barrier film 111 is a layer having a function of suppressing waterand hydrogen of layers below the barrier film 111 from diffusing upward.Furthermore, the barrier film 111 preferably has low oxygenpermeability. Furthermore, the barrier film 111 may have an opening or aplug for electrically connecting an electrode or a wiring provided overthe barrier film 111 to an electrode or a wiring provided below thebarrier film 111. Here, a film to which water and hydrogen are lesslikely to diffuse refers to a film which has lower water and hydrogenpermeability than silicon oxide or the like that is generally used as aninsulating film, for example. Furthermore, a film having low oxygenpermeability refers to a film having lower oxygen permeability thansilicon oxide or the like that is generally used as an insulating film.

Here, it is preferable that hydrogen, water, and the like in the layersbelow the barrier film 111 be reduced as much as possible.Alternatively, degasification is preferably reduced. Hydrogen or watermight become a factor that causes changes in the electricalcharacteristics of an oxide semiconductor. Furthermore, hydrogen orwater diffusing from the layers below the barrier film 111 to the layersover the barrier film 111 can be suppressed by the barrier film 111;however, the hydrogen or water might diffuse to the layers thereoverthrough an opening, a plug, or the like provided in the barrier film111.

To reduce hydrogen and water contained in the layers below the barrierfilm 111 or to reduce degasification, heat treatment for removing thehydrogen and the water contained in the layers below the barrier film111 or for reducing degasification is preferably performed before theformation of the barrier film 111 or immediately after the formation ofan opening for forming a plug in the barrier film 111. The heattreatment is preferably performed at as high a temperature as possibleas long as the heat resistance of the conductive films and the like inthe semiconductor device and the electrical characteristics of thetransistor are not degraded. Specifically, the temperature may be, forexample, 450° C. or higher, preferably 490° C. or higher, furtherpreferably 530° C. or higher, or may be 650° C. or higher. It ispreferable that the heat treatment be performed under an inert gasatmosphere or a reduced pressure atmosphere for 1 hour or longer,preferably 5 hours or longer, further preferably 10 hours or longer. Thetemperature is determined in consideration of the heat resistance of thematerials of wirings or electrodes positioned in the layer below thebarrier film 111; for example, in the case where the heat resistance ofthe materials is low, the temperature is preferably 550° C. or lower,600° C. or lower, 650 or lower, or 800° C. or lower. Such heat treatmentmay be performed at least once but is preferably performed more thanonce.

It is preferable that the amount of released hydrogen molecules of theinsulating film provided in the layer below the barrier film 111, whichis measured by thermal desorption spectroscopy analysis (also referredto as TDS analysis), at a substrate surface temperature of 400° C. belower than or equal to 130%, preferably lower than or equal to 110% ofthat at 300° C. Alternatively, it is preferable that the amount ofreleased hydrogen molecules measured by TDS analysis at a substratesurface temperature of 450° C. be lower than or equal to 130%,preferably lower than or equal to 110% of that at 350° C.

Water and hydrogen contained in the barrier film 111 itself are alsopreferably reduced. Alternatively, degasification is preferably reduced.For example, a material where the amount of released hydrogen molecules(M/z=2) at a substrate surface temperature of 20° C. to 600° C., whichis measured by TDS analysis, is less than 2×10¹⁵/cm², preferably lessthan 1×10¹⁵/cm², and further preferably less than 5×10¹⁴/cm² ispreferably used for the barrier film 111. Alternatively, a materialwhere the amount of released water molecules (M/z=18) at a substratesurface temperature of 20° C. to 600° C., which is measured by TDSanalysis, is less than 1×10¹⁶/cm², preferably less than 5×10¹⁵/cm², andfurther preferably less than 2×10¹²/cm² is preferably used for thebarrier film 111.

Furthermore, the heat treatment can also serve as treatment (alsoreferred to as hydrogenation treatment) for terminating, with hydrogen,dangling bonds (also referred to as dangling bonds) of silicon used inthe semiconductor layers of the transistor 130 a, the transistor 130 b,the transistor 230 a, and the transistor 230 b. By the hydrogenationtreatment, part of hydrogen contained in the gate insulating films ofthe transistor 130 a, the transistor 130 b, the transistor 230 a, andthe transistor 230 b or other insulating films formed in the layersbelow the barrier film 111 is released and diffused to the semiconductorlayers of the transistor 130 a, the transistor 130 b, the transistor 230a, and the transistor 230 b to terminate dangling bonds in silicon, sothat the reliability of the transistor 130 a, the transistor 130 b, thetransistor 230 a, and the transistor 230 b can be improved.

As a material that can be used for the barrier film 111, a single layeror a stacked layer of an insulating film containing what is called ahigh-k material such as aluminum oxide, hafnium oxide, tantalum oxide,zirconium oxide, lead zirconate titanate (PZT), strontium titanate(SrTiO₃), or (Ba, Sr)TiO₃ (BST) can be used. Alternatively, aluminumoxide, bismuth oxide, germanium oxide, niobium oxide, silicon oxide,titanium oxide, tungsten oxide, yttrium oxide, zirconium oxide, orgallium oxide may be added to the insulating films, for example.Alternatively, the insulating film may be subjected to nitridingtreatment to be an oxynitride film. A layer of silicon oxide, siliconoxynitride, or silicon nitride may be stacked over the insulating film.Gallium oxide or the like is given. Aluminum oxide is particularlypreferable because of its excellent barrier property against water orhydrogen.

For the barrier film 111, a layer of a material which is not permeatedwith water or hydrogen easily and a layer containing another insulatingmaterial may be stacked and used. For example, a layer containingsilicon oxide or silicon oxynitride, a layer containing a metal oxide,and the like may be stacked.

Furthermore, for the barrier film 111, a material that is not permeatedwith oxygen easily is preferably used. The above-described materialshave excellent barrier properties against oxygen as well as hydrogen andwater. The use of such materials can suppress diffusion of oxygenreleased when an insulating film 114 j is heated to the layers below thebarrier film 111. Consequently, the amount of oxygen that is releasedfrom the insulating film 114 j and is likely to be supplied to thesemiconductor layers of the transistor Ta_j and the transistor Tb_j canbe increased.

In this manner, the concentration of hydrogen or water contained in eachlayer provided in the layer below the barrier film 111 is reduced or thehydrogen or water is removed or degasification is suppressed, andfurthermore, diffusion of hydrogen or water to the transistor Ta_j andthe transistor Tb_j is suppressed by the barrier film 111. Thus, theamount of hydrogen and water contained in the insulating film 114 j andthe layers in the transistor Ta_j and the transistor Tb_j can beextremely low. For example, the concentration of hydrogen contained inthe insulating film 114 j, semiconductor layers 101 j of the transistorTa_j and the transistor Tb_j, or a gate insulating film 102 j can bereduced to, for example, lower than 5×10¹⁸ cm⁻³, preferably lower than1×10¹⁸ cm⁻³, further preferably lower than 3×10¹⁷ cm⁻³.

With the above structure, high reliability can be obtained in both ofthe peripheral circuit 500 including the transistor using silicon as thesemiconductor layer and the memory cell array 300 including thetransistor using an oxide semiconductor as the semiconductor layer,which can achieve a semiconductor device having extremely highreliability.

Note that in the above description, the example in which the peripheralcircuit 500 includes the transistor using silicon as the semiconductorlayer is shown; however, the peripheral circuit 500 may include both thetransistor using silicon as the semiconductor layer and the transistorusing an oxide semiconductor as the semiconductor layer. In that case,for example, after the barrier film 111 is formed over the transistorusing silicon as the semiconductor layer, the transistor using an oxidesemiconductor as the semiconductor layer may be stacked over the barrierfilm 111 to form the peripheral circuit 500. Furthermore, the memorycell array 300 including the transistor using an oxide semiconductor asthe semiconductor layer may be stacked over the peripheral circuit 500.

Here, an example of a circuit configuration in which a p-channel typetransistor using silicon as the semiconductor layer and an n-channeltype transistor using an oxide semiconductor as the semiconductor layerare used and which can be used for the peripheral circuit 500 isdescribed.

[CMOS Circuit]

A circuit diagram in FIG. 26A shows a configuration of what is called aCMOS circuit in which a p-channel type transistor 2200 and an n-channeltype transistor 2100 are connected to each other in series and in whichgates of them are connected to each other. Note that in drawings, atransistor in which a second semiconductor material is used is denotedby a symbol “OS”.

[Analog Switch]

Furthermore, a circuit diagram shown in FIG. 26B shows a configurationin which sources of the transistor 2100 and the transistor 2200 areconnected to each other and drains thereof are connected to each other.With such a configuration, the transistors can function as what iscalled an analog switch.

Stacked-Layer Structure Example 1

Next, a stacked-layer structure example of a semiconductor deviceincluding the memory cell array 300 and the peripheral circuit 500 isdescribed with reference to FIG. 6. The memory cell array 300 isprovided over the peripheral circuit 500. The memory cell array 300includes the memory cell CL. The memory cell CL includes c sub memorycells SCL_j (j is a natural number of 1 to c). In FIG. 6, astacked-layer structure example of the sub memory cell SCL_1 and the submemory cell SCL_2 is shown; the sub memory cell SCL_3 to the sub memorycell SCL_c are further stacked in order over the sub memory cell SCL_2,though not shown. Note that FIG. 1B is referred to for a circuit diagramof the memory cell array 300.

The sub memory cell SCL_j includes the transistor Ta_j, the transistorTb_j, and the capacitor Ca_j. The transistor Ta_j and the transistorTb_j include an oxide semiconductor material. Here, the barrier film 111is preferably provided between the transistor Tb_j and the peripheralcircuit 500 in the case where j=1.

Furthermore, the capacitor Ca_j is provided over the transistor Tb_j.Furthermore, at least part of the capacitor Ca_j is preferably providedto overlap with the transistor Tb_j. Here, a conductive layer 151 jwhich is one of terminals of the capacitor Ca_j is electricallyconnected to a gate electrode 203 j of the transistor Tb_j through aplug 141 j. Furthermore, an insulating film 216 j is provided betweenthe transistor Tb_j and the capacitor Ca_j.

The transistor Ta_j is provided over the capacitor Ca_j. At least partof the transistor Ta_j is preferably provided to overlap with thecapacitor Ca_j. Here, the gate electrode 203 j and the conductive layer151 j are electrically connected to a conductive layer 104 j_bfunctioning as the source electrode or the drain electrode of thetransistor Ta_j through the plug 141 j and a plug 144 j. An insulatingfilm 115 j included in the capacitor Ca_j is provided between theconductive layer 151 j and a conductive layer 152 j and formscapacitance. Furthermore, an insulating film 156 j is provided betweenthe capacitor Ca_j and the transistor Ta_j. Furthermore, an insulatingfilm 116 j is provided over the transistor Ta_j.

A conductive layer 204 j_b functioning as the source electrode or thedrain electrode of the transistor Tb_j is electrically connected to thesource line SL. Here, a plug 143 j, a conductive layer 154 j, a plug 146j, a plug 148 j, and the like may function as the source line SL.

A conductive layer 204 j_a functioning as the source electrode or thedrain electrode of the transistor Tb_j is electrically connected to aconductive layer 104 j_a functioning as the source electrode or thedrain electrode of the transistor Ta_j through a plug 142 j, aconductive layer 153 j, a plug 145 j, and the like. Furthermore, theconductive layer 204 j_a and the conductive layer 104 j_a areelectrically connected to the bit line BL. Here, the plug 142 j, theconductive layer 153 j, the plug 145 j, and the like may function as thebit line BL.

Furthermore, the sub memory cell SCL_j and an adjacent sub memory cellSCL_a share the plug 142 j, the conductive layer 153 j, the plug 145 j,the plug 147 j, and the like. The sub memory cell SCL_a is electricallyconnected to the bit line BL through the plug 142 j, the conductivelayer 153 j, the plug 145 j, the plug 147 j, and the like. Similarly,the sub memory cell SCL_j and an adjacent sub memory cell SCL_β sharethe plug 143 j, the conductive layer 154 j, the plug 146 j, the plug 148j, and the like. The sub memory cell SCL_β is electrically connected tothe source line SL through the plug 143 j, the conductive layer 154 j,the plug 146 j, the plug 148 j, and the like. The integration degree ofthe memory cell array 300 can be increased by sharing plugs and aconductive layer in this manner.

Here, the sub memory cell SCL_a is a sub memory cell included in amemory cell CL_α adjacent to the memory cell CL. Here, an adjacentmemory cell refers to a memory cell whose value of x or y in x and ycoordinates (x, y) shown in FIG. 1A differs by one. For example, thememory cell CL_α is positioned on coordinates where an x coordinate issmaller by one than that in the memory cell CL. Similarly, for example,the memory cell CL_β is positioned on coordinates where an x coordinateis larger by one than that in the memory cell CL.

The transistor Tb_j included in the sub memory cell SCL_j and atransistor Tb_a included in the sub memory cell SCL_a share theconductive layer 204 j_a. That is, the conductive layer 204 j_afunctions as one of the source electrode and the drain electrode of thetransistor Tb_j, and functions as one of a source electrode and a drainelectrode of the transistor Tb_α. Furthermore, the transistor Tb_j and atransistor Tb_β included in the sub memory cell SCL_β share theconductive layer 204 j_b. That is, the conductive layer 204 j_bfunctions as the other of the source electrode and the drain electrodeof the transistor Tb_j, and functions as one of a source electrode and adrain electrode of the transistor Tb_β. The integration degree of thememory cell array 300 can be increased by sharing a conductive layer inthis manner.

The transistor Ta_j included in the sub memory cell SCL_j and atransistor Ta a included in the sub memory cell SCL_a share theconductive layer 104 j_a. That is, the conductive layer 104 j_afunctions as one of the source electrode and the drain electrode of thetransistor Ta_j, and functions as one of a source electrode and a drainelectrode of the transistor Ta_α. The integration degree of the memorycell array 300 can be increased by sharing a conductive layer in thismanner.

Here, structures of the transistor Ta_j and the transistor Tb_j aredescribed. The transistor Ta_j and the transistor Tb_j are transistorsincluding an oxide semiconductor. An example of a transistor structurethat can be used for the transistor Ta_j and the transistor Tb_j isshown in FIGS. 7A to 7E.

FIG. 7B is a top view of a transistor structure that can be used for thetransistors Ta_j and Tb_j, FIG. 7A shows a cross section alongdashed-dotted line A-B shown in FIG. 7B, and FIG. 7C shows a crosssection along dashed-dotted line C-D shown in FIG. 7B. Here, thetransistor Ta_j is shown as an example, but the transistor Tb_j can havea similar structure. Furthermore, for each component of the transistorTb_j, the description of the transistor Ta_j is referred to. Forexample, for a semiconductor layer 201 j, the semiconductor layer 101 jis referred to. Furthermore, for a gate insulating film 202 j, the gateinsulating film 102 j is referred to. Furthermore, for a gate electrode203 j, a gate electrode 103 j is referred to. Furthermore, for aconductive layer 204 j_a and a conductive layer 204 j_b, the conductivelayer 104 j_a and the conductive layer 104 j_b are referred to.Furthermore, for an insulating film 214 j, the insulating film 114 j isreferred to. Furthermore, for a conductive layer 205 j, a conductivelayer 105 j is referred to.

The transistor Ta_j includes the semiconductor layer 101 j in contactwith the upper surface of the insulating film 114 j, the conductivelayer 104 j_a and the conductive layer 104 j_b, the gate insulating film102 j over the semiconductor layer 101 j, and the gate electrode 103 joverlapping with the semiconductor layer 101 j with the gate insulatingfilm 102 j positioned therebetween. Furthermore, an insulating film isprovided to cover the transistor Ta_j. Here, although not shown, threelayers of an insulating film 112 j, an insulating film 113 j, and theinsulating film 116 j can be stacked and used as the insulating filmcovering the transistor Ta_j, for example. The insulating film 112 j,the insulating film 113 j, and the insulating film 116 j are describedin a manufacturing method example to be described later. One of theconductive layer 104 j_a and the conductive layer 104 j_b functions asthe source electrode and the other functions as the drain electrode.

The transistor Ta_j shown in FIGS. 7A to 7C includes a semiconductorlayer 101 j_a, a semiconductor layer 101 j_b in contact with the uppersurface of the semiconductor layer 101 j_a, the conductive layer 104 j_aand the conductive layer 104 j_b that are in contact with the uppersurface of the semiconductor layer 101 j_b and are apart from each otherin a region overlapping with the semiconductor layer 101 j_b, thesemiconductor layer 101 j_c in contact with the upper surface of thesemiconductor layer 101 j_b, the gate insulating film 102 j over thesemiconductor layer 101 j_c, and the gate electrode 103 j overlappingwith the semiconductor layer 101 j_b with the gate insulating film 102 jand the semiconductor layer 101 j_c positioned therebetween.Furthermore, the transistor Ta_j includes the conductive layer 105 jfunctioning as a second gate electrode. The semiconductor layer 101 j_ais provided between the insulating film 114 j and the semiconductorlayer 101 j_b. Furthermore, the semiconductor layer 101 j_c is providedbetween the semiconductor layer 101 j_b and the gate insulating film 102j. Furthermore, the conductive layer 104 j_a and the conductive layer104 j_b are in contact with the upper surface of the semiconductor layer101 j_b.

Furthermore, as in cross-sectional views shown in FIG. 7D and FIG. 7Ecorresponding to FIG. 7A and FIG. 7C, respectively, the insulating film114 j may have a protruding portion and the semiconductor layer 101 j_aand the semiconductor layer 101 j_b may be provided over the protrudingportion.

Furthermore, as shown in FIGS. 18A to 18C, for example, in a crosssection in FIG. 18C, a structure where the gate insulating film 102 jcovers an end portion of the semiconductor layer 101 j_c may be used.

The semiconductor layer 101 j of the transistor Ta_j is preferablyprovided over the insulating film 114 j. The insulating film 114 jpreferably includes oxide. In particular, an oxide material from whichpart of oxygen is released by heating is preferably included. It ispreferable that oxide containing oxygen more than oxygen of thestoichiometric composition be used. In the case where an oxidesemiconductor is used as the second semiconductor material, oxygenreleased from the insulating film 114 j is supplied to the oxidesemiconductor, so that oxygen vacancies in the oxide semiconductor canbe reduced. Consequently, changes in the electrical characteristics ofthe second transistor can be reduced and the reliability of the secondtransistor can be improved.

It is preferable that the upper surface of the insulating film 114 j beplanarized by planarization treatment described above.

An oxide material from which part of oxygen is released by heating ispreferably used for the insulating film 114 j.

As the oxide material from which oxygen is released by heating, oxidecontaining oxygen more than oxygen of the stoichiometric composition ispreferably used. Part of oxygen is released by heating from an oxidefilm containing oxygen more than oxygen of the stoichiometriccomposition. The oxide film containing oxygen more than oxygen of thestoichiometric composition is an oxide film that releases oxygenmolecules the amount of which is more than or equal to 1.0×10¹⁸atoms/cm³, preferably more than or equal to 3.0×10²⁰ atoms/cm³ inthermal desorption spectroscopy (TDS: Thermal Desorption Spectroscopy)analysis at a film surface temperature of higher than or equal to 100°C. and lower than or equal to 700° C., preferably higher than or equalto 100° C. and lower than or equal to 500° C.

For example, as such a material, a material containing silicon oxide orsilicon oxynitride is preferably used. Alternatively, metal oxide can beused. As the metal oxide, aluminum oxide, aluminum oxynitride, galliumoxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafniumoxide, hafnium oxynitride, or the like can be used. Note that in thisspecification, silicon oxynitride refers to a material that containsmore oxygen than nitrogen in its composition, and silicon nitride oxiderefers to a material that contains more nitrogen than oxygen in itscomposition.

Similarly, the semiconductor layer 201 j included in the transistor Tb_jis preferably provided over the insulating film 214 j.

Furthermore, the transistor Ta_j preferably includes the conductivelayer 105 j. The conductive layer 105 j preferably functions as thesecond gate of the transistor Ta_j.

As shown in FIGS. 19A to 19C, the transistor Tb_j includes thesemiconductor layer 201 j in contact with the upper surface of theinsulating film 214 j, the conductive layer 204 j_a and the conductivelayer 204 j_b, the gate insulating film 202 j over the semiconductorlayer 201 j, and the gate electrode 203 j overlapping with thesemiconductor layer 201 j with the gate insulating film 202 j providedtherebetween. Furthermore, an insulating film 212 j, an insulating film213 j, and the insulating film 216 j are provided to cover thetransistor Tb_j. One of the conductive layer 204 j_a and the conductivelayer 204 j_b functions as the source electrode and the other functionsas the drain electrode.

Furthermore, the transistor Tb_j may include the conductive layer 205 j.The conductive layer 205 j may function as the second gate of thetransistor Tb_j.

Here, in the case where voltages are applied between electrodes of theconductive layer 105 j included in the transistor Ta_j and theconductive layer 205 j included in the transistor Tb_j, the voltages maybe different. Here, a difference between a voltage applied to theconductive layer 105 j and a source voltage is referred to as Vbg_1, anda difference between a voltage applied to the conductive layer 205 j anda source voltage is referred to as Vbg_2. By increasing absolute valuesof Vbg_1 and Vbg_2, lower off-state current can be achieved in somecases. On the other hand, when the absolute values of Vbg_1 and Vbg_2are increased too much, a rising voltage of on-state current isincreased, so that the transistors can be operated at low currentvoltage. Therefore, considering that off-state current of the transistorTa_j is preferably lower than that of the transistor Tb_j, the absolutevalue of Vbg_1 may be larger than the absolute value of Vbg_2, forexample. By making the absolute values of the Vbg_1 and Vbg_2 differ inthis manner, retention characteristics of the semiconductor device canbe improved, and power consumption can be reduced. Furthermore, theoperation speed of the semiconductor device can be increased.

Note that the semiconductor layer 101 j may be formed of a single layer,or may be formed with a stacked-layer structure of the semiconductorlayer 101 j_a, the semiconductor layer 101 j_b, and the semiconductorlayer 101 j_c as in the example illustrated in FIGS. 7A to 7E.Similarly, the semiconductor layer 201 j may be formed of a single layeror may be formed with a stacked-layer structure of a semiconductor layer201 j_a, a semiconductor layer 201 j_b, and a semiconductor layer 201j_c.

For the insulating film 112 j, as in the case of the barrier film 111, amaterial to which water and hydrogen do not easily diffuse is preferablyused. Furthermore, in particular, a material that is not permeated withoxygen easily is preferably used for the insulating film 112 j. Notethat the insulating film 112 j may have a stacked-layer structure of twoor more layers. In this case, for example, the insulating film 112 j mayhave a stacked-layer structure of two layers in which, for example,silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide,aluminum nitride, or the like is used for the lower layer. Furthermore,a material to which water and hydrogen do not easily diffuse ispreferably used for the upper layer, as in the case of the barrier film111. Furthermore, an insulating film provided in the lower layer may bean insulating film from which oxygen is released by heating as in thecase of the insulating film 114 j, so that oxygen is supplied also fromabove the semiconductor layer 101 j through the gate insulating film 102j.

By covering the semiconductor layer 101 j with the insulating film 112 jincluding a material that is not permeated with oxygen easily, releaseof oxygen from the semiconductor layer 101 j to a portion over theinsulating film 112 j can be suppressed. Furthermore, oxygen releasedfrom the insulating film 114 j can be confined below the insulating film112 j; thus, the amount of oxygen to be supplied to the semiconductorlayer 101 j can be increased.

Furthermore, the insulating film 112 j that is not permeated with wateror hydrogen easily can suppress entry of water or hydrogen, which is animpurity for an oxide semiconductor, from the outside so that change inelectrical characteristics of the transistor Ta_j can be suppressed anda highly reliable transistor can be achieved.

Note that an insulating film from which oxygen is released by heatinglike the insulating film 114 j may be provided below the insulating film112 j to supply oxygen also from a portion over the semiconductor layer101 j through the gate insulating film 102 j.

For the insulating film 212 j over the transistor Tb_j, the descriptionof the insulating film 112 j may be referred to.

As shown in FIG. 7A, a side surface of the semiconductor layer 101 j_bof the transistor Ta_j is in contact with the conductive layer 104 j_aand the conductive layer 104 j_b. Furthermore, the semiconductor layer101 j_b can be electrically surrounded by an electric field of the gateelectrode 103 j (a structure in which a semiconductor is electricallysurrounded by an electric field of a conductor is referred to as asurrounded channel (s-channel) structure). Therefore, a channel isformed in the entire semiconductor layer 101 j_b (bulk) in some cases.In the s-channel structure, a large amount of current can flow between asource and a drain of a transistor, so that a current at the time ofconduction (on-state current) can be increased.

It can be said that the s-channel structure is suitable for aminiaturized transistor because a high on-state current can be obtained.A semiconductor device including the transistor can have a highintegration degree and high density because the transistor can beminiaturized. For example, the transistor includes a region where thechannel length is preferably less than or equal to 40 nm, furtherpreferably less than or equal to 30 nm, still further preferably lessthan or equal to 20 nm, and the transistor includes a region where thechannel width is preferably less than or equal to 40 nm, furtherpreferably less than or equal to 30 nm, still further preferably lessthan or equal to 20 nm.

Note that the channel length refers to, for example, a distance betweena source (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor (or aportion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other or a region where achannel is formed in a top view of the transistor. Note that in onetransistor, channel lengths in all regions do not necessarily have thesame value. In other words, the channel length of one transistor is notfixed to one value in some cases. Therefore, in this specification, thechannel length is any one of values, the maximum value, the minimumvalue, or the average value in a region where a channel is formed.

A channel width refers to, for example, the length of a portion where asource and a drain face each other in a region where a semiconductor (ora portion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other, or a region where achannel is formed. Note that in one transistor, channel widths in allregions do not necessarily have the same value. In other words, achannel width of one transistor is not fixed to one value in some cases.Therefore, in this specification, a channel width is any one of values,the maximum value, the minimum value, or the average value in a regionwhere a channel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is higher than the proportion of a channel region formedin a top surface of the semiconductor in some cases. In that case, aneffective channel width obtained when a channel is actually formed isgreater than an apparent channel width shown in the top view.

Meanwhile, in a transistor having a three-dimensional structure, aneffective channel width is difficult to measure in some cases. Forexample, estimation of an effective channel width from a design valuerequires an assumption that the shape of a semiconductor is known.Therefore, in the case where the shape of a semiconductor is not knownaccurately, it is difficult to measure an effective channel widthaccurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a “surroundedchannel width (SCW: Surrounded Channel Width)” in some cases. Further,in this specification, in the case where the term “channel width” issimply used, it denotes a surrounded channel width or an apparentchannel width in some cases. Alternatively, in this specification, inthe case where the term “channel width” is simply used, it denotes aneffective channel width in some cases. Note that the values of a channellength, a channel width, an effective channel width, an apparent channelwidth, a surrounded channel width, and the like can be determined byobtaining and analyzing a cross-sectional TEM image and the like.

Note that in the case where electron field-effect mobility, a currentvalue per channel width, and the like of a transistor are obtained bycalculation, a surrounded channel width is used for the calculation insome cases. In those cases, a value different from one in the case wherean effective channel width is used for the calculation may be obtained.

It is preferable that the semiconductor layer 101 j and thesemiconductor layer 201 j include a semiconductor having a wider energygap than silicon. The semiconductor layer 101 j preferably includes anoxide semiconductor. A semiconductor material having a wider energy gapand a lower carrier density than silicon is preferably used becauseoff-state current of the transistor can be reduced.

The use of such a material for the semiconductor layer makes it possibleto provide a highly reliable transistor in which a change in theelectrical characteristics is suppressed.

Note that a preferable mode and a formation method of an oxidesemiconductor that can be used for the semiconductor layer are describedin detail in an embodiment later.

Note that in this specification and the like, the carrier density of asubstantially intrinsic oxide semiconductor layer is lower than1×10¹⁷/cm³, lower than 1×10¹⁵/cm³, lower than 1×10¹³/cm³, lower than8×10¹¹/cm³, lower than 1×10¹¹/cm³, or lower than 1×10¹⁰/cm³, and ishigher than or equal to 1×10⁻⁹/cm³. With a highly purified intrinsicoxide semiconductor layer, stable electric characteristics can beimparted to the transistor.

When an In—Ga—Zn-based oxide having an atomic ratio of In:Ga:Zn=1:1:1,2:1:3, 3:1:2, or 4:2:3 is used for the semiconductor layer 101 j_b, forexample, an In—Ga—Zn-based oxide having an atomic ratio ofIn:Ga:Zn=1:3:2, 1:3:4, 1:3:6, 1:6:4, 1:6:8, 1:6:10, 1:9:6, 1:2:3, or thelike can be used for the semiconductor layer 101 j_a or thesemiconductor layer 101 j_c. Note that the atomic ratio of each of thesemiconductor layer 101 j_b, the semiconductor layer 101 j_a, and thesemiconductor layer 101 j_c may vary within a range of ±20% of theabove-described atomic ratio as an error. For the semiconductor layer101 j_a and the semiconductor layer 101 j_c, materials with the samecomposition or materials with different compositions may be used.

Furthermore, when an In-M-Zn-based oxide is used for the semiconductorlayer 101 j_b, a target for forming the semiconductor film serving asthe semiconductor layer 101 j_b is preferably an oxide containing metalelements in the atomic ratio satisfying the following: given that theatomic ratio of metal elements in the target is In:M:Zn=x₁:y₁:z₁, avalue of x₁/y₁ is greater than or equal to ⅓ and less than or equal to6, preferably greater than or equal to 1 and less than or equal to 6,and z₁/y₁ is greater than or equal to ⅓ and less than or equal to 6,preferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is less than or equal to 6, a CAAC-OS film to bedescribed later is easily formed. Typical examples of the atomic ratioof the metal elements in the target are In:M:Zn=1:1:1, 2:1:3, 3:1:2, andthe like.

Furthermore, when an In-M-Zn-based oxide is used for the semiconductorlayer 101 j_a and the semiconductor layer 101 j_c, a target for formingoxide semiconductor films to be the semiconductor layer 101 j_a and thesemiconductor layer 101 j_c is preferably an oxide containing metalelements at the atomic ratio satisfying the following: given that theatomic ratio of metal elements in the target is In:M:Zn=x₂:y₂:z₂, x₂/y₂is less than x₁/y₁ and a value of z₂/y₂ is greater than or equal to ⅓and less than or equal to 6, preferably greater than or equal to 1 andless than or equal to 6. Note that when z₂/y₂ is less than or equal to6, a CAAC-OS film to be described later is easily formed. Typicalexamples of the atomic ratio of the metal elements in the target areIn:M:Zn=1:3:4, 1:3:6, 1:3:8, and the like.

In the case where an oxide semiconductor is formed by a sputteringmethod, a film having an atomic ratio different from the atomic ratio ofthe target may be formed. In particular, the atomic ratio of zinc in afilm is smaller than the atomic ratio thereof in the target in somecases. Specifically, the proportion of zinc in the film is approximately40 atomic % to 90 atomic % of that of zinc in the target in some cases.

For the conductive layer 104 j_a and the conductive layer 104 j_b, asingle-layer structure or a stacked-layer structure of metal such asaluminum, titanium, chromium, nickel, copper, yttrium, zirconium,molybdenum, silver, tantalum, or tungsten or an alloy containing it asits main component is used. For example, a single-layer structure of analuminum film containing silicon, a two-layer structure in which analuminum film is stacked over a titanium film, a two-layer structure inwhich an aluminum film is stacked over a tungsten film, a two-layerstructure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a two-layer structure in which acopper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a tungsten film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, a three-layer structure in whicha molybdenum film or a molybdenum nitride film, an aluminum film or acopper film, and a molybdenum film or a molybdenum nitride film arestacked in this order, and the like can be given. Note that atransparent conductive material containing indium oxide, tin oxide, orzinc oxide may be used.

The gate insulating film 102 j may be formed with a single layer or astacked layer of, for example, silicon oxide, silicon oxynitride,silicon nitride oxide, aluminum oxide, hafnium oxide, gallium oxide,Ga—Zn-based metal oxide, silicon nitride, and the like.

Furthermore, for the gate insulating film 102 j, a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), or yttrium oxide may be used.

Furthermore, the gate insulating film 102 j can be formed using an oxideinsulating film such as aluminum oxide, magnesium oxide, silicon oxide,silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, ortantalum oxide, a nitride insulating film such as silicon nitride,silicon nitride oxide, aluminum nitride, or aluminum nitride oxide, or afilm of a mixed material of these.

Furthermore, as the gate insulating film 102 j, an oxide insulating filmthat contains more oxygen than oxygen in the stoichiometric compositionis preferably used, as in the case of the insulating film 114 j.

Note that when a specific material is used for the gate insulating film,electrons are trapped in the gate insulating film under the specificconditions and the threshold voltage can be increased. For example, likea stacked-layer film of silicon oxide and hafnium oxide, part of thegate insulating film uses a material having a lot of electron trapstates, such as hafnium oxide, aluminum oxide, and tantalum oxide, andthe state where the potential of the gate electrode is higher than thatof the source electrode or the drain electrode is kept for one second ormore, typically one minute or more at a higher temperature (atemperature higher than the operating temperature or the storagetemperature of the semiconductor device, or a temperature of 125° C. orhigher and 450° C. or lower, typically a temperature of 150° C. orhigher and 300° C. or lower); thus, electrons are moved from thesemiconductor layer to the gate electrode, and some of the electrons aretrapped by the electron trap states.

In the transistor in which a necessary amount of electrons is trapped bythe electron trap states in this manner, the threshold voltage isshifted in the positive direction. By controlling the voltage of thegate electrode, the amount of electrons to be trapped can be controlled,and thus the threshold voltage can be controlled. Furthermore, thetreatment for trapping the electrons may be performed in themanufacturing process of the transistor.

For example, the treatment is preferably performed at any step beforefactory shipment, such as after the formation of a wire metal connectedto the source electrode or the drain electrode of the transistor, afterthe preceding process (wafer processing), after a wafer-dicing step,after packaging, or the like. In either case, it is preferable that thesemiconductor device be not exposed to temperatures of 125° C. or higherfor one hour or more after the treatment.

The gate electrode 103 j can be formed using, for example, a metalselected from aluminum, chromium, copper, tantalum, titanium,molybdenum, and tungsten; an alloy containing the above-described metalas a component; an alloy containing above-described metals incombination; or the like. Furthermore, one or more metals selected frommanganese and zirconium may be used. Alternatively, a semiconductortypified by polycrystalline silicon doped with an impurity element suchas phosphorus, or a silicide such as nickel silicide may be used.Furthermore, the gate electrode 103 j may have a single-layer structureor a stacked-layer structure of two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film or a tungstennitride film, a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in this order, and thelike can be given. Alternatively, an alloy film or a nitride film inwhich aluminum and one or more selected from titanium, tantalum,tungsten, molybdenum, chromium, neodymium, and scandium are combined maybe used.

Furthermore, for the gate electrode 103 j, a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded can be used. It is also possible to have a stacked-layer structureof the above light-transmitting conductive material and the above metal.

Furthermore, for the conductive layer 105 j, a material similar to thatused for the gate electrode 103 j may be used.

Furthermore, an In—Ga—Zn-based oxynitride semiconductor film, anIn—Sn-based oxynitride semiconductor film, an In—Ga-based oxynitridesemiconductor film, an In—Zn-based oxynitride semiconductor film, aSn-based oxynitride semiconductor film, an In-based oxynitridesemiconductor film, a film of metal nitride (such as InN or ZnN), or thelike may be provided between the gate electrode 103 j and the gateinsulating film 102 j. These films have a work function higher than orequal to 5 eV, preferably higher than or equal to 5.5 eV, which ishigher than the electron affinity of an oxide semiconductor; thus, thethreshold voltage of a transistor including an oxide semiconductor canbe shifted in the positive direction, and a switching element havingwhat is called normally-off characteristics can be obtained. Forexample, in the case of using an In—Ga—Zn-based oxynitride semiconductorfilm, an In—Ga—Zn-based oxynitride semiconductor film having a highernitrogen concentration than at least the semiconductor layer 101 j,specifically an In—Ga—Zn-based oxynitride semiconductor film having anitrogen concentration higher than or equal to 7 at. %, is used.

Note that the structure shown in FIG. 7A is an example in which the edgeportions of the gate insulating film 102 j and the semiconductor layer101 j_c are processed so as to be substantially aligned with each other,and the gate electrode 103 j is processed so as to be positioned on theinner side than the gate insulating film; alternatively, the edgeportions of the gate insulating film 102 j, the semiconductor layer 101j_c, and the gate electrode 103 j may be processed so as to besubstantially aligned with one another. Alternatively, the edge portionsof the gate insulating film 102 j, the semiconductor layer 101 j_c, andthe gate electrode may be processed so as not to be aligned with oneanother.

Furthermore, examples of different structures of the transistor Ta_j andthe transistor Tb_j are briefly described using FIG. 18D, FIG. 18E, FIG.19D, and FIG. 19E. Here, although the transistor Ta_j is described, asimilar structure can be used for the transistor Tb_j.

In forming the semiconductor layer 101 j_a and the semiconductor layer101 j_b, a conductive film 104 is formed, a resist mask is formed, theconductive film 104 is etched, and then a semiconductor layer to be thesemiconductor layer 101 j_a and a semiconductor layer to be thesemiconductor layer 101 j_b are formed by etching. After that, theconductive film 104 is processed again to form the semiconductor layer104 j_a and the semiconductor layer 104 j_b, so that a structure asshown in FIG. 18D can be obtained.

Alternatively, the semiconductor layer 101 j_c may be provided incontact with lower surfaces of the conductive layer 104 j_a and theconductive layer 104 j_b as shown in FIG. 18E. Such a structure enablesfilms used for the semiconductor layer 101 j_a, the semiconductor layer101 j_b, and the semiconductor layer 101 j_c to be formed successivelywithout contact with the air and therefore can reduce defects at eachinterface.

Alternatively, the transistor Ta_j may have a structure shown in FIG.19D. In the example shown in FIG. 19D, opening portions are provided inthe insulating film 116 j and plugs are formed, so that a sourceelectrode and a drain electrode are obtained. Furthermore, thetransistor Ta_j may include the insulating film 112 j and the insulatingfilm 113 j below the insulating film 116 j.

Alternatively, low-resistance regions may be provided in thesemiconductor layer 101 j as shown in FIG. 19E. First, a semiconductorfilm to be the semiconductor layer 101 j is formed over the insulatingfilm 114 j, and then, a resist mask or the like is formed and etching isperformed to form the semiconductor layer 101 j. Next, an insulatingfilm to be the gate insulating film 102 j and a conductive film to bethe gate electrode 103 j are formed, a resist mask or the like isformed, and then etching is performed to form the gate electrode 103 jand the gate insulating film 102 j.

Then, a low-resistance region 171 j_a and a low-resistance region 171j_b are formed. A semiconductor layer having high carrier density haslow resistance. As ways to increase the carrier density, for example,addition of an impurity, formation of oxygen vacancies, and the like canbe given. For example, as a way to increase the carrier density, anelement may be added by ion implantation. As the element which can beused, one or more kinds selected from argon, boron, carbon, magnesium,aluminum, silicon, phosphorus, calcium, scandium, titanium, vanadium,chromium, manganese, iron, cobalt, nickel, gallium, germanium, arsenic,yttrium, zirconium, niobium, molybdenum, indium, tin, lanthanum, cerium,neodymium, hafnium, tantalum, and tungsten are preferably added. Thelow-resistance region 171 j_a and the low-resistance region 171 j_b are,for example, regions each containing any of the above impurities in thesemiconductor layer 101 j at a concentration of higher than or equal to5×10¹⁹ atoms/cm³, preferably higher than or equal to 1×10²⁰ atoms/cm³,further preferably higher than or equal to 2×10²⁰ atoms/cm³, stillfurther preferably higher than or equal to 5×10²⁰ atoms/cm³.

There is a possibility that, for example, unnecessary hydrogen can betrapped in such a low-resistance region. The trap of unnecessaryhydrogen in the low-resistance layer can reduce the hydrogenconcentration of the channel region, and as a result, the transistorTa_j can have favorable characteristics.

In the examples shown in FIG. 7A to 7E, FIGS. 18A to 18E, and FIGS. 19Ato 19E, the structure in which the semiconductor layer 101 j_a and thesemiconductor layer 101 j_c are provided in contact with thesemiconductor layer 101 j_b is described; however, a structure withoutone or both of the semiconductor layer 101 j_a and the semiconductorlayer 101 j_c may be employed.

The above is the description of the transistor Ta_j and the transistorTb_j.

The insulating film 116 j covering the transistor Ta_j functions as aplanarization layer which covers an uneven shape of a layer thereunder.Furthermore, the insulating film 113 j may have a function as aprotective film when the insulating film 116 j is formed. The insulatingfilm 113 j is not necessarily provided.

Similarly, the insulating film 216 j covering the transistor Tb_jfunctions as a planarization layer which covers an uneven shape of alayer thereunder. Furthermore, the insulating film 213 j may have afunction as a protective film when the insulating film 216 j is formed.The insulating film 213 j is not necessarily provided.

Furthermore, the description of the insulating film 116 j may bereferred to for the insulating film 156 j covering the capacitor Ca_j.

The plug 147 j and the like that are electrically connected to theconductive layer 104 j_a and the like are embedded in the insulatingfilm 112 j, the insulating film 113 j, and the insulating film 116 j.

Furthermore, the plug 141 j and the like that are electrically connectedto the gate electrode 203 j, the conductive layer 151 j, and the likeare embedded in the insulating film 212 j, the insulating film 213 j,and the insulating film 216 j.

Furthermore, as illustrated in FIG. 6, the insulating film 137 whichcontains the same material as the barrier film 111 may be provided overthe insulating film 136 containing hydrogen. This structure caneffectively suppresses water or hydrogen remaining in the insulatingfilm 136 containing hydrogen from diffusing upward. In that case, heattreatment for removing water or hydrogen may performed twice or more intotal: before formation of the insulating film 137, and after formationof the insulating film 137 and before formation of the barrier film 111.

A conductive material such as a metal material, an alloy material, or ametal oxide material can be used as a material for the plug 141 j to theplug 148 j, the conductive layer 151 j to the conductive layer 154 j,and the like. It is particularly preferable to use a high-melting-pointmaterial that has both heat resistance and conductivity, such astungsten or molybdenum, and it is particularly preferable to usetungsten. Furthermore, a material such as titanium nitride or titaniumand another material may be stacked. For example, use of titaniumnitride or titanium can improve adhesion with the opening portion.Furthermore, it is preferable that the plug 141 j to the plug 148 j, theconductive layer 151 j to the conductive layer 154 j, and the like beprovided so as to be embedded in the insulating films and the uppersurfaces thereof be each planarized.

Oxide semiconductor layers are repeatedly stacked to form asemiconductor layer in the transistor Ta_j and the transistor Tb_j,whereby c sub memory cells SCL can be stacked as shown in FIG. 1A, FIG.6, and the like. Accordingly, the capacity per area can be increased.

In a conventional transistor using silicon, germanium, or a compoundthereof, in particular, in an element having a short channel length, itis preferable that a gate electric field be strengthened in order toreduce a short-channel effect, and the thickness of a gate insulatingfilm is preferably reduced in order to strengthen the gate electricfield.

In contrast, a transistor using an oxide semiconductor film is anaccumulation-type transistor in which electrons are majority carriers.Therefore, the influence of DIBL (Drain-Induced Barrier Lowering) as ashort-channel effect is smaller than in an inversion-type transistorhaving a pn junction. In other words, the transistor using an oxidesemiconductor film is resistant to a short-channel effect.

The transistor using an oxide semiconductor film can have a thicker gateinsulating film than a conventional transistor including silicon or thelike because of its high resistance to a short-channel effect. Forexample, a gate insulating film with a thickness as large asapproximately 10 nm can be used in a minute transistor having a channellength and a channel width of 50 nm or less. Here, when the gateinsulating film is thick, parasitic capacitance can be small. Thus,dynamic behavior of a circuit may be improved. Furthermore, when thegate insulating film is thick, leakage current and power consumption maybe low.

Furthermore, a drain electric field is strengthened with a reduction inthe channel length; thus, a reduction in reliability due to hot-carrierdegradation noticeably occurs in a conventional transistor using siliconor the like, in particular, having a short channel width. In contrast,in some cases, avalanche breakdown or the like is less likely to occurin the case of using an oxide semiconductor than in a conventionaltransistor using silicon or the like, because, for example, an oxidesemiconductor has a wide energy gap (e.g., 2.5 eV or more in an oxidesemiconductor containing indium, gallium, and zinc) and thus electronsare less likely to be excited, and the effective mass of a hole islarge. Therefore, it may be possible to inhibit hot-carrier degradationor the like due to avalanche breakdown.

When the gate insulating film is thick, the withstand voltage of thegate insulating film can be increased, so that the transistor can bedriven at a higher gate voltage. In addition, hot-carrier degradation isinhibited, whereby the transistor can be driven at a high drain voltagewithout lengthening of the channel length. Thus, the reliability of thetransistor in a circuit supplied with high voltage can be increased, andthe channel length can be reduced, so that the integration degree of thecircuit can be increased.

In a transistor using an intrinsic or substantially intrinsic oxidesemiconductor film, when the distance between the source electrode andthe drain electrode is sufficiently short, the energy at the bottom ofthe valence band is low because of the electric fields of the source andthe drain, so that the energy at the bottom of the valence band is closeto the Fermi level. This phenomenon is called a Conduction Band LoweringEffect (CBL effect). Owing to the CBL effect, a drain current starts toflow at a low gate voltage that is close to 0 V in the Vg-Idcharacteristics, so that the driving voltage of the transistor may bereduced.

Here, a CAAC-OS film is preferably used as the oxide semiconductor film.It is preferable that the CAAC-OS film have a high CAAC proportion. Anincrease in the CAAC proportion enables, for example, a reduction ininfluence of carrier scattering in the transistor, resulting in highfield-effect mobility. Furthermore, furthermore, the influence of agrain boundary can be reduced; as a result, variation in on-statecharacteristics of the transistor can be reduced. Thus, a highlyreliable semiconductor device can be obtained. Furthermore, use of thetransistor with small variation can reduce driving voltage to reducepower consumption. In addition, for example, a CAAC-OS film having a lowdensity of defects can be obtained. Alternatively, a CAAC-OS film with asmall amount of impurities can be obtained. A reduction in the densityof defects makes it possible to obtain extremely low off-state currentcharacteristics, for example. The CAAC-OS film is described later.

Here, the semiconductor layer 101 j_b included in the transistor Ta_jand the semiconductor layer 201 j_b included in the transistor Tb_j maybe formed using different materials. For example, in the case where anIn-M-Zn oxide is used for the semiconductor layer 101 j_b and thesemiconductor layer 201 j_b, materials differing in the atomic ratio ofindium to an element M and zinc may be used.

Furthermore, transistors having different structures may be used for thetransistor Ta_j and the transistor Tb_j. Furthermore, the transistorTa_j and the transistor Tb_j may have different channel widths.Similarly, the transistor Ta_j and the transistor Tb_j may havedifferent channel lengths.

Furthermore, the semiconductor layer 101 j included in the transistorTa_j and the semiconductor layer 201 j included in the transistor Tb_jpreferably include an oxide semiconductor including In, the element M,and Zn. The case is described in which the atomic ratio of In to M andZn in the oxide semiconductor included in the semiconductor layer 101 jsatisfies In:M:Zn=a:b:c, whereas the atomic ratio of In to M and Zn inthe oxide semiconductor included in the semiconductor layer 201 jsatisfies In:M:Zn=d:e:f. Here, for example, it is preferable thata/(a+b+c) be smaller than d/(d+e+f). When the percentage of indiumcontent is increased, overlaps of the s orbitals are increased;therefore, the mobility of oxide in which the percentage of indiumcontent is high is higher than that of oxide in which the percentage ofindium content is low. Therefore, when oxide having a high indiumcontent is used as the oxide semiconductor film, carrier mobility can beincreased. However, when the percentage of indium content is decreased,off-state current can be reduced in some cases, which is preferable.

Stacked-Layer Structure Example 2

Furthermore, a stacked-layer structure shown in FIG. 8 is an example ofthe stacked-layer structure of the semiconductor device which isdifferent from that shown in FIG. 6. The semiconductor device shown inFIG. 8 includes a memory cell array 300 and the peripheral circuit 500.Note that although FIG. 8 shows layers stacked up to j=4, layers of submemory cells of j=5 or more may be stacked in reality; the number ofstacked layers is preferably as large as possible because theintegration degree of the memory can be increased accordingly. For acircuit diagram of the memory cell array 300 shown in FIG. 8, forexample, FIG. 4 is referred to. Here, in FIG. 8, the interface betweenfilms and the like are partly omitted to make the diagram easy tounderstand.

Here, a layer 291 shown in FIG. 8 includes transistors. Furthermore, alayer 292 and a layer 293 shown in FIG. 8 include conductive layers.Furthermore, a layer 294 shown in FIG. 8 includes transistors.Furthermore, a layer 295 and a layer 296 shown in FIG. 8 includeconductive layers. Top views of the layer 291 to the layer 296 shown inFIG. 8 are shown in FIG. 10.

A transistor, a capacitor, a transistor, a transistor, a capacitor, anda transistor are stacked in this order in FIG. 6, whereas a transistor,a capacitor, a transistor, and a capacitor are stacked in this order inFIG. 8 by arranging sub memory cells in a staggered configuration. Thus,the process can be simplified.

FIG. 9 is an enlarged view of part of FIG. 8. The capacitor Ca_j isprovided over the transistor Tb_j. Furthermore, at least part of thecapacitor Ca_j is preferably provided to overlap with the transistorTb_j. Furthermore, part of the capacitor Ca_j may be provided to overlapwith a transistor included in an adjacent sub memory cell. In an exampleshown in FIG. 9, for example, a capacitor Ca_3 included in the submemory cell SCL_3 is provided to overlap with a transistor Ta_2 includedin the sub memory cell SCL_2, so that the integration degree of thememory cell array 300 can be increased.

Furthermore, the sub memory cell SCL_2 and the sub memory cell SCL_3have structures symmetrical to each other in FIG. 9. With such astructure, the sub memory cells can be arranged in a staggeredconfiguration, and the capacitor can be formed over the transistor inthe adjacent sub memory cell, whereby the integration degree can beincreased. Furthermore, by the arrangement in a staggered configuration,the process can be more simplified than that of FIG. 6. That is, the submemory cells stacked one above the other include transistors formed inthe same layer. Specifically, for example, the transistor Ta_j (j=m−1)included in the sub memory cell SCL_j (j=m−1) and the transistor Tb_j(j=m) included in the sub memory cell SCL_j (j=m) are formed in the samelayer. That is, for example, the transistor Ta_j (j=m−1) and thetransistor Tb_j (j=m) are formed over and in contact with a firstinsulating film. That is, for example, the semiconductor layer 101 j(j=m−1) included in the transistor Ta_j (j=m−1) and the semiconductorlayer 201 j (j=m) included in the transistor Tb_j (j=m) are formed overand in contact with the first insulating film. Furthermore, the memorycell array 300 includes the first insulating film in contact with thesemiconductor layer 101 j (j=m−1) and the semiconductor layer 201 j(j=m). Furthermore, for example, a second insulating film is formed tobe over and in contact with the gate electrode 103 j (j=m−1) included inthe transistor Ta_j (j=m−1) and the gate electrode 203 j (j=m) includedin the transistor Tb_j (j=m). Furthermore, the memory cell array 300includes the second insulating film in contact with the gate electrode103 j (j=m−1) and the gate electrode 203 j (j=m). Here, m is a naturalnumber of 2 or more.

In this manner, transistors included in adjacent sub memory cellsarranged one above the other are partly formed in the same layer,whereby the memory cell array 300 can be manufactured by fewer steps.Manufacturing by fewer steps can increase yield. Furthermore, becausethe number of layers to be stacked can be reduced, parasitic capacitancein the whole circuit can be reduced.

In addition, with the increase in the number of stacked layers, thefilms are possibly peeled or broken due to film stress of insulatingfilms, conductive films, semiconductor films, and the like. Therefore,when the number of layers to be stacked is reduced, the malfunction ofthe semiconductor device due to film peeling or the like can besuppressed.

The conductive layer 151 j which is one of the terminals of thecapacitor Ca_j is electrically connected to the gate electrode 203 j ofthe transistor Tb_j through the plug 141 j.

The transistor Ta_j is provided over the capacitor Ca_j. Furthermore, atleast part of the transistor Ta_j is preferably provided to overlap withthe capacitor Ca_j. Here, the gate electrode 203 j and the conductivelayer 151 j are electrically connected to the conductive layer 104 j_bfunctioning as the source electrode or the drain electrode of thetransistor Ta_j through the plug 141 j and the plug 144 j. Theinsulating film 115 j included in the capacitor Ca_j is provided betweenthe conductive layer 151 j and the conductive layer 152 j and formscapacitance.

The conductive layer 204 j_b functioning as the source electrode or thedrain electrode of the transistor Tb_j is electrically connected to thesource line SL.

The conductive layer 204 j_a functioning as the source electrode or thedrain electrode of the transistor Tb_j is electrically connected to theconductive layer 104 j_a functioning as the source electrode or thedrain electrode of the transistor Ta_j through the plug 142 j and thelike. Furthermore, the conductive layer 204 j_a and the conductive layer104 j_a are electrically connected to the bit line BL.

Furthermore, the sub memory cell SCL_j and an adjacent sub memory cellpreferably share a conductive layer, a plug, and the like. For example,the conductive layer 204 j_a functioning as the source electrode or thedrain electrode of the transistor Tb_j included in the sub memory cellSCL_2 and the source electrode or the drain electrode of the transistorTa_j included in an adjacent sub memory cell SC_1 are shared.Furthermore, the conductive layer 104 j_a functioning as the sourceelectrode or the drain electrode of the transistor Ta_j included in thesub memory cell SCL_2 and the source electrode or the drain electrode ofthe transistor Tb_j included in the sub memory cell SCL_3 are shared.The integration degree of the memory cell array 300 can be increased bysharing the conductive layer in this manner.

Furthermore, the transistor 198 and the transistor 199 shown in FIG. 8are what is called dummy transistors not included in any of sub memorycells. Therefore, the transistor 198 and the transistor 199 aregenerally considered unnecessary for a memory cell array. However,providing the transistor 198 and the transistor 199 enables the layer299 to be formed using the same mask as that for the layer 294 or thelike. Therefore, the number of masks that are necessary can be reduced,leading to the cost reduction. Furthermore, in a lithography process,particularly in the case where a minute pattern is formed, a change inthe distance between lines or the like may lead to a variation infinished line width or the like in some cases. Therefore, in the casewhere a lithography process is used for the manufacture of the memorycell array 300, the layer 294 and the layer 299 are preferably formedusing the same mask, because conditions such as a distance betweenpatterns can be shared and a minute pattern is easily formed in thelayer 294 and the layer 299, and furthermore, sizes of semiconductorlayers, conductive layers, gate electrodes, and the like of finishedtransistors can be uniform. Furthermore, a distance between transistors,between a transistor and a wiring, and between wirings does not varymuch; therefore, interference of an electric field such as capacitybetween wirings can be uniform, and a variation in characteristicsbetween sub memories can be reduced in some cases. Note that a gateelectrode and a conductive layer which are included in each of thetransistor 198 and the transistor 199 are not necessarily connected toanother transistor or a wiring. In that case, the formation of anunnecessary plug and wiring can be omitted. Furthermore, although notshown in FIG. 8, it is preferable that a dummy transistor be similarlyprovided in the uppermost layer of the memory cell array 300.

Here, the transistor 198 and the transistor 199 which are dummytransistors may be connected to part of the write word line WWL, theread word line RWL, the bit line BL, the floating node FN, and thesource line SL, or may be connected to none of them.

For example, the transistor 198 and the transistor 199 which are dummytransistors may be connected to only the source line SL, or may beconnected to the source line SL and the bit line BL.

Furthermore, for example, the transistor 198 and the transistor 199which are dummy transistors are connected to one of terminals of acapacitor, and the other of the terminals of the capacitor is notnecessarily connected to the write word line WWL.

Here, in FIG. 8, the layer 299 and the layer 294 preferably include thesame number of transistors, for example. Alternatively, transistorshaving substantially the same shape are preferably included.

Furthermore, in FIG. 8, the layer 299 and the layer 294 preferablyinclude the same number of semiconductor layers, for example.Alternatively, semiconductor layers having substantially the same shapeare preferably included.

Furthermore, in FIG. 8, the layer 299 and the layer 294 preferablyinclude the same number of gate electrodes, for example. Alternatively,gate electrodes having substantially the same shape are preferablyincluded.

Stacked-Layer Structure Example 3

Furthermore, a stacked-layer structure shown in FIGS. 11A to 11C is anexample of the stacked-layer structure of the semiconductor device whichis different from that shown in FIG. 6 and FIG. 8. FIG. 11A shows a topview of a sub memory cell included in a memory cell array 300.Furthermore, FIG. 11B shows a cross section along A-A′, a cross sectionalong A′-B, and a cross section along B-B′ shown in FIG. 11A.Furthermore, FIG. 11C shows a cross section along C-C′ shown in FIG.11A. The semiconductor device shown in FIG. 11B includes the memory cellarray 300 and the peripheral circuit 500. Note that although FIG. 11shows layers stacked up to j=2, layers of sub memory cells of j=3 ormore may be stacked in reality; the number of stacked layers ispreferably as large as possible because the integration degree of amemory can be increased accordingly. For a circuit diagram of the memorycell array 300 shown in FIG. 8, for example, FIG. 1B is referred to.

In the structure shown in FIG. 11B, one of the terminals of thecapacitor Ca_j can serve as the gate electrode 203 j; therefore, theprocess can be simplified.

Stacked-Layer Structure Example 4

Furthermore, a stacked-layer structure shown in FIG. 12 is an example ofthe stacked-layer structure of the semiconductor device which isdifferent from that shown in FIG. 6, FIG. 8, and FIGS. 11A to 11C.Furthermore, the stacked-layer structure shown in FIG. 12 is an exampleof the stacked-layer structure of the semiconductor device which isdifferent from that shown in FIG. 6. The semiconductor device shown inFIG. 12 includes a memory cell array 300 and the peripheral circuit 500.Note that although FIG. 12 shows layers stacked up to j=4, layers of submemory cells of j=5 or more may be stacked in reality; the number ofstacked layers is preferably as large as possible because theintegration degree of a memory can be increased accordingly. For acircuit diagram of the memory cell array 300 shown in FIG. 12, forexample, FIG. 4 is referred to.

The memory cell array 300 shown in FIG. 12 is the same as that in FIG. 8in that the sub memory cells are arranged in a staggered configuration.Of sub memory cells SCL1 to SCL4 shown in FIG. 12, SCL2 and SCL4 differfrom those in the other structures in that the gate electrode 203 jlocated below the semiconductor layer 101 j of the transistor Tb_j, whatis called a bottom gate, is electrically connected to the capacitorCa_j. Furthermore, the stacking order of two transistors and a capacitoralso differs from that in the other structures in that the capacitorCa_j is located over the transistor Ta_j and that the transistor Tb_j islocated over the capacitor Ca_j.

Meanwhile, in the SCL_1 and the SCL_3, the gate electrode 203 jpositioned over the semiconductor layer 101 j of the transistor Tb_j iselectrically connected to the capacitor Ca_j. Furthermore, regarding thestacking order of two transistors and a capacitor, the capacitor Ca_j ispositioned over the transistor Tb_j and the transistor Ta_j ispositioned over the capacitor Ca_j. That is, in the example shown inFIG. 12, the structure of the sub memory cell SCL_j varies depending onwhether j is an odd number or an even number. Furthermore, in theexample shown in FIG. 12, the transistor Ta_j and the transistor Tb_jare not necessarily formed in the same layer, which is preferablebecause the manufacture is simple in the case where the semiconductorlayer 101 j included in the transistor Ta_j and the semiconductor layer201 j included in the transistor Tb_j are formed using oxidesemiconductor layers differing in the constituent elements or the atomicratio, for example.

The above is the description of the structural example.

Manufacturing Method Example

An example of a method for manufacturing the semiconductor devicedescribed in the above structure example is described below withreference to FIGS. 13A to 13E, FIGS. 14A to 14C, FIGS. 15A to 15C, andFIGS. 16A and 16B.

First, the semiconductor substrate 131 is prepared. As the semiconductorsubstrate 131, for example, a single crystal silicon substrate(including a p-type semiconductor substrate or an n-type semiconductorsubstrate), a compound semiconductor substrate containing siliconcarbide or gallium nitride, or the like can be used. Furthermore, an SOIsubstrate may be used as the semiconductor substrate 131. The case wheresingle crystal silicon is used for the semiconductor substrate 131 isdescribed below.

Next, an element isolation layer (not illustrated) is formed in thesemiconductor substrate 131. The element isolation layer may be formedby a LOCOS (Local Oxidation of Silicon) method, a STI (Shallow TrenchIsolation) method, mesa isolation, or the like.

In the case where a p-type transistor and an n-type transistor areformed on the same substrate, an n-well or a p-well may be formed inpart of the semiconductor substrate 131. For example, a p-well may beformed by adding an impurity element imparting p-type conductivity, suchas boron, to an n-type semiconductor substrate 131, and an n-typetransistor and a p-type transistor may be formed on the same substrate.

Next, an insulating film to be the gate insulating film 134 is formedover the semiconductor substrate 131. For example, a surface of thesemiconductor substrate 131 is oxidized to form a silicon oxide film.Alternatively, a stacked-layer structure of a silicon oxide film and asilicon oxynitride film may be formed by forming silicon oxide by athermal oxidation method and then nitriding a surface of a film of thesilicon oxide by nitridation treatment. Alternatively, silicon oxide,silicon oxynitride, a metal oxide such as tantalum oxide, hafnium oxide,hafnium silicate oxide, zirconium oxide, aluminum oxide, or titaniumoxide, which is a high dielectric constant material (also referred to asa high-k material), rare-earth oxide such as lanthanum oxide, or thelike may be used.

The insulating film may be formed by deposition using a sputteringmethod, a CVD (Chemical Vapor Deposition) method (including a thermalCVD method, an MOCVD (Metal Organic CVD) method, a PECVD (PlasmaEnhanced CVD) method, or the like), an MBE (Molecular Beam Epitaxy)method, an ALD (Atomic Layer Deposition) method, a PLD (Pulsed LaserDeposition) method, or the like.

Next, a conductive film to be the gate electrode 135 is formed. It ispreferable that the conductive film be formed using a metal selectedfrom tantalum, tungsten, titanium, molybdenum, chromium, niobium, andthe like, or an alloy material or a compound material including any ofthe metals as its main component. Alternatively, polycrystalline siliconto which an impurity such as phosphorus is added can be used.Alternatively, a stacked-layer structure including a film of metalnitride and a film of the above metal may be used. As metal nitride,tungsten nitride, molybdenum nitride, or titanium nitride can be used.When the metal nitride film is provided, adhesiveness of the metal filmcan be increased, and separation can be prevented.

The conductive film can be formed by a sputtering method, an evaporationmethod, a CVD method (including a thermal CVD method, an MOCVD method, aPECVD method, and the like), or the like. Furthermore, a thermal CVDmethod, an MOCVD method, or an ALD method is preferable in order toreduce plasma damage.

Next, a resist mask is formed over the conductive film by a lithographymethod or the like and unnecessary portions of the conductive film areremoved. Then, the resist mask is removed; thus, the gate electrode 135can be formed.

Here, a method for processing a film to be processed is described. Inthe case of finely processing a film to be processed, a variety of fineprocessing techniques can be used. For example, a method may be used inwhich a resist mask formed by a photolithography method or the like maybe subjected to slimming treatment. Alternatively, a method may be usedin which a dummy pattern is formed by a photolithography method or thelike, the dummy pattern is provided with a sidewall and is then removed,and a film to be processed is etched using the remaining sidewall as aresist mask. In order to achieve a high aspect ratio, anisotropic dryetching is preferably used for etching of a film to be processed.Alternatively, a hard mask formed of an inorganic film or a metal filmmay be used.

As light used to form the resist mask, light with an i-line (with awavelength of 365 nm), light with a g-line (with a wavelength of 436nm), light with an h-line (with a wavelength of 405 nm), or light inwhich these are mixed can be used. Alternatively, ultraviolet light, KrFlaser light, ArF laser light, or the like can be used. Exposure may beperformed by liquid immersion exposure technique. As the light for theexposure, extreme ultra-violet light (EUV: Extreme Ultra-violet) orX-rays may be used. Instead of the light for the exposure, an electronbeam can be used. It is preferable to use extreme ultra-violet light,X-rays, or an electron beam because extremely minute processing can beperformed. Note that in the case of performing exposure by scanning of abeam such as an electron beam, a photomask is not needed.

An organic resin film having a function of improving adhesion betweenthe film to be processed and a resist film may be formed before theresist film serving as a resist mask is formed. The organic resin filmcan be formed by covering a step in a layer lower than that to planarizea surface by a spin coating method or the like, and thus can reducevariation in thickness of the resist mask provided in a layer upper thanthe organic resin film. In the case where a particularly minute processis performed, a material functioning as an anti-reflection film againstlight for exposure is preferably used for the organic resin film. As anorganic resin film with such a function, a BARC (Bottom Anti-ReflectionCoating) film and the like can be given as examples. The organic resinfilm may be removed at the same time as the removal of the resist maskor after the resist mask is removed.

After the gate electrode 135 is formed, a sidewall covering the sidesurface of the gate electrode 135 may be formed. The sidewall can beformed in such a manner that an insulating film thicker than the gateelectrode 135 is formed and subjected to anisotropic etching so thatonly a portion of the insulating film on the side surface of the gateelectrode 135 remains.

FIGS. 13A to 13E illustrate an example in which etching of the gateinsulating film is not performed at the time of formation of thesidewall; however, the insulating film to be the gate insulating film134 may be etched at the same time as formation of the sidewall. In thiscase, the gate insulating film 134 is provided below the gate electrode135 and the sidewall.

Next, an element imparting n-type conductivity, such as phosphorus, oran element imparting p-type conductivity, such as boron, is added to aregion of the semiconductor substrate 131 where the gate electrode 135(and the sidewall) is not provided. A schematic cross-sectional view atthis stage corresponds to FIG. 13A.

Next, the insulating film 136 is formed, and then, first heat treatmentis performed to activate the aforementioned element which impartsconductivity.

For the insulating film 136, a stacked layer or a single layer of, forexample, silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitrideoxide, aluminum nitride, or the like may be used. The insulating film136 can be formed by a sputtering method, a CVD method (including athermal CVD method, an MOCVD method, a PECVD method, and the like), anMBE method, an ALD method, a PLD method, or the like. In particular, itis preferable that the insulating film be formed by a CVD method,further preferably a plasma CVD method because coverage can be improved.Furthermore, a thermal CVD method, an MOCVD method, or an ALD method ispreferable in order to reduce plasma damage.

The first heat treatment can be performed at a temperature higher thanor equal to 400° C. and lower than the strain point of the substrate inan inert gas atmosphere of a rare gas, a nitrogen gas, or the like or ina reduced-pressure atmosphere.

At this stage, the transistor 130 a, the transistor 130 b, thetransistor 230 a, and the transistor 230 b are formed.

Next, the insulating film 137 is formed, so that a cross section shownin FIG. 13A is obtained. Next, the insulating film 138 is formed.

The insulating film 137 can be formed using the material that can beused for the insulating film 136, and is preferably formed using siliconnitride containing oxygen and hydrogen (SiNOH) because the amount ofhydrogen released by heating can be increased. Alternatively, theinsulating film 138 can be formed using the material that can be usedfor the insulating film 136, and is preferably formed using siliconoxide with high step coverage that is formed by reacting TEOS(Tetra-Ethyl-Ortho-Silicate), silane, or the like with oxygen, nitrousoxide, or the like.

The insulating film 137 and the insulating film 138 can be formed by asputtering method, a CVD method (including a thermal CVD method, anMOCVD method, a PECVD method, and the like), an MBE method, an ALDmethod, a PLD method, or the like, for example. In particular, it ispreferable that the insulating film be formed by a CVD method, furtherpreferably a plasma CVD method because coverage can be further improved.Furthermore, a thermal CVD method, an MOCVD method, or an ALD method ispreferable in order to reduce plasma damage.

Next, the upper surface of the insulating film 138 is planarized by aCMP method or the like. As the insulating film 138, a planarization filmmay be used. At this time, a CMP method or the like is not necessarilyused for planarization. The planarization film can be formed by, forexample, an atmospheric pressure CVD method, a coating method, or thelike. As a film which can be formed by an atmospheric pressure CVDmethod, BPSG (Boron Phosphorus Silicate Glass) and the like can be givenas examples. Furthermore, as a film which can be formed by a coatingmethod, HSQ (hydrogen silsesquioxane) and the like can be given asexamples.

After that, second heat treatment is performed so that dangling bonds inthe semiconductor layer 132 are terminated by hydrogen released from theinsulating film 137. Furthermore, by the second heat treatment, waterand hydrogen in the layers are released; thus, the water content and thehydrogen content can be reduced.

The second heat treatment can be performed under the conditions given asan example in the above description of the stacked-layer structure. Forexample, the conditions described for the first heat treatment or thelike can be used.

Next, the barrier film 111 is formed. The barrier film 111 can be formedby a sputtering method, a CVD method (including a thermal CVD method, anMOCVD method, a PECVD method, and the like), an MBE method, an ALDmethod, a PLD method, or the like, for example. In particular, it ispreferable that the insulating film be formed by a CVD method, furtherpreferably a plasma CVD method because coverage can be improved.Furthermore, a thermal CVD method, an MOCVD method, or an ALD method ispreferable in order to reduce plasma damage.

Next, a conductive film to be the conductive layer 105 j, the conductivelayer 153 j, and the like is formed over the barrier film 111. Theconductive film to be the conductive layer 105 j, the conductive layer153 j, and the like can be formed by, for example, a sputtering method,a CVD method (including a thermal CVD method, an MOCVD method, a PECVDmethod, and the like), an MBE method, an ALD method, a PLD method, orthe like. In particular, it is preferable that the insulating film beformed by a CVD method, further preferably a plasma CVD method becausecoverage can be improved. Furthermore, a thermal CVD method, an MOCVDmethod, or an ALD method is preferable in order to reduce plasma damage.

Next, a resist mask is formed, and an unnecessary portion of theconductive film to be the conductive layer 105 j, the conductive layer153 j, and the like is removed by etching. After that, the resist maskis removed, so that the conductive layer 105 j, the conductive layer 153j, and the like are formed.

Then, the insulating film 214 j is formed. The insulating film 214 j canbe formed by a sputtering method, a CVD method (including a thermal CVDmethod, an MOCVD method, a PECVD method, and the like), an MBE method,an ALD method, a PLD method, or the like, for example. In particular, itis preferable that the insulating film be formed by a CVD method,further preferably a plasma CVD method because coverage can be improved.Furthermore, a thermal CVD method, an MOCVD method, or an ALD method ispreferable in order to reduce plasma damage. Note that for theinsulating film 214 j, the description of the insulating film 114 j maybe referred to.

To make the insulating film 214 j contain excess oxygen, the insulatingfilm 214 j may be deposited in an oxygen atmosphere, for example.Alternatively, a region containing excess oxygen may be formed byintroducing oxygen into the insulating film 214 j that has been formed,or both of the methods may be combined.

For example, oxygen (at least including any of oxygen radicals, oxygenatoms, and oxygen ions) is introduced into the insulating film 214 jthat has been formed, whereby a region containing excess oxygen isformed. As a method for introducing oxygen, an ion implantation method,an ion doping method, a plasma immersion ion implantation method, plasmatreatment, or the like can be used.

A gas containing oxygen can be used for oxygen introduction treatment.As the gas containing oxygen, oxygen, dinitrogen monoxide, nitrogendioxide, carbon dioxide, carbon monoxide, and the like can be used.Furthermore, a rare gas may be included in the gas containing oxygen forthe oxygen introduction treatment. Furthermore, hydrogen or the like maybe included. For example, a mixed gas of carbon dioxide, hydrogen, andargon may be used.

After the insulating film 214 j is formed, planarization treatment usinga CMP method or the like may be performed in order to increase theplanarity of the upper surface thereof.

Next, a plug for connection with the source electrode, the drainelectrode, or the like of the transistor Tb_j may be formed over theconductive layer 153 j and the like. First, an opening portion isprovided in the insulating film 214 j (see FIG. 13B). Next, a conductivefilm 153 to be the plug is formed to fill the opening portion (see FIG.13C). For the formation of the conductive film 153, the conductive filmto be the conductive layer 105 j, or the like may be referred to, forexample.

Next, planarization treatment is performed on the conductive film 153 sothat the surface of the insulating film 214 j is exposed, whereby theplug 148 j and the like are formed (see FIG. 13D).

Next, the transistors such as the transistor Tb_j are formed over theinsulating film 214 j. Note that the method for manufacturing thetransistor Tb_j can be referred to for a method for manufacturing thetransistor Ta_j.

A semiconductor film to be the semiconductor layer 201 j_a and the likeand a semiconductor film to be the semiconductor layer 201 j_b and thelike are sequentially formed. The semiconductor films are preferablyformed successively without contact with the air. A semiconductor to bethe semiconductor layer 201 j_a and a semiconductor to be thesemiconductor layer 201 j_b can be formed by a sputtering method, a CVDmethod, an MBE method, a PLD method, an ALD method, or the like.

Note that in the case where In—Ga—Zn oxide layers are formed as thesemiconductor to be the semiconductor layer 201 j_a and the like and thesemiconductor to be the semiconductor layer 201 j_b and the like by anMOCVD method, trimethylindium, trimethylgallium, dimethylzinc, and thelike may be used as the source gases. Note that the source gases are notlimited to the above-described combination, and triethylindium or thelike may be used instead of trimethylindium. Furthermore,triethylgallium or the like may be used instead of trimethylgallium.Furthermore, diethylzinc or the like may be used instead ofdimethylzinc.

After the oxide film and the semiconductor film are formed, fourth heattreatment is preferably performed. The heat treatment may be performedat a temperature higher than or equal to 250° C. and lower than or equalto 650° C., preferably higher than or equal to 300° C. and lower than orequal to 500° C., in an inert gas atmosphere, in an atmospherecontaining an oxidizing gas at 10 ppm or more, or under reducedpressure. Alternatively, the heat treatment may be performed in an inertgas atmosphere and then performed in an atmosphere containing anoxidizing gas at 10 ppm or more in order to compensate released oxygen.The heat treatment may be performed directly after the formation of thesemiconductor films or may be performed after the semiconductor filmsare processed into the island-shaped semiconductor layers 201 j_a and201 j_b and the like. By the heat treatment, oxygen can be supplied tothe semiconductor films from the insulating film 214 j and the oxidefilm; thus, oxygen vacancies in the semiconductor films can be reduced.

Then, a resist mask is formed, and an unnecessary portion is removed byetching. Then, the resist mask is removed; thus a stacked-layerstructure including the island-shaped semiconductor layers 201 j_a and201 j_b and the like can be formed (see FIG. 13E). Note that part of theinsulating film 214 j might be etched in the etching of thesemiconductor films to reduce the thickness of the insulating film 214 jin a region which is not covered with the semiconductor layer 201 j_a,the semiconductor layer 201 j_b, and the like. For this reason, it ispreferable that the insulating film 214 j be formed thick in advance soas not to be removed by the etching.

After that, a conductive film 204 j is formed. The conductive film 204 jcan be formed by a sputtering method, a CVD method (including a thermalCVD method, an MOCVD method, a PECVD method, and the like), an MBEmethod, an ALD method, a PLD method, or the like, for example. Inparticular, it is preferable that the insulating film be formed by a CVDmethod, further preferably a plasma CVD method because coverage can beimproved. Furthermore, a thermal CVD method, an MOCVD method, or an ALDmethod is preferable in order to reduce plasma damage.

Next, a resist mask is formed, and an unnecessary portion of theconductive film 204 j is removed by etching. After that, the resist maskis removed; thus, the conductive layer 204 j_a, the conductive layer 204j_b, and the like are formed (see FIG. 14A). Here, in some cases, partof the upper portions of the semiconductor layer 201 j_b and theinsulating film 114 j and the like are etched in the etching of theconductive film and the thickness of a portion where the conductivelayer 204 j_a and the conductive layer 204 j_b do not overlap therewithis reduced. For this reason, it is preferable that the semiconductorfilm and the like to be the semiconductor layer 201 j_b be formed thickin advance in consideration of the etching depth.

Next, the gate insulating film 202 j and the semiconductor layer 201 j_care formed. The gate insulating film 202 j and the semiconductor layer201 j_c may be formed in such a manner that after films to be the gateinsulating film 202 j and the semiconductor layer 201 j_c are formed, aresist mask is formed, and processing is performed by etching. Next, aconductive film to be the gate electrode 203 j is formed. After that, aresist mask is formed, the conductive film is processed by etching, andthe resist mask is then removed; thus, the gate electrode 203 j isformed (see FIG. 14B). A semiconductor to be the semiconductor layer 101j_c can be formed by a sputtering method, a CVD method, an MBE method, aPLD method, an ALD method, or the like.

Note that in the case where an In—Ga—Zn oxide layer is formed as thesemiconductor to be the semiconductor layer 101 j_c by an MOCVD method,trimethylindium, trimethylgallium, dimethylzinc, and the like may beused as source gases. Note that the source gases are not limited to theabove-described combination, and triethylindium or the like may be usedinstead of trimethylindium. Furthermore, triethylgallium or the like maybe used instead of trimethylgallium. Furthermore, diethylzinc or thelike may be used instead of dimethylzinc.

At this stage, the transistors such as the transistor Tb_j are formed.

Then, the insulating film 212 j is formed. The insulating film 212 j canbe formed by a sputtering method, a CVD method (including a thermal CVDmethod, an MOCVD method, a PECVD method, and the like), an MBE method,an ALD method, a PLD method, or the like, for example. In particular, itis preferable that the insulating film be formed by a CVD method,further preferably a plasma CVD method because coverage can be improved.Furthermore, a thermal CVD method, an MOCVD method, or an ALD method ispreferable in order to reduce plasma damage.

After the insulating film 212 j is formed, fifth heat treatment ispreferably performed. By the heat treatment, oxygen can be supplied fromthe insulating film 214 j and the like to the semiconductor layer 201 jto reduce oxygen vacancies in the semiconductor layer 201 j.Furthermore, at this time, oxygen released from the insulating film 214j is blocked by the barrier film 111 and the insulating film 212 j anddoes not diffuse to a layer lower than the barrier film 111 and a layerupper than the insulating film 212 j; therefore, the oxygen can beeffectively confined. Thus, the amount of oxygen to be supplied to thesemiconductor layer 201 j can be increased, so that oxygen vacancies inthe semiconductor layer 201 j can be effectively reduced.

Furthermore, the insulating film 212 j may have a stacked-layerstructure of two or more layers. In this case, for example, theinsulating film 212 j may have a stacked-layer structure of two layersin which, for example, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride,aluminum nitride oxide, aluminum nitride, or the like is used for thelower layer. Furthermore, a material to which water and hydrogen do noteasily diffuse is preferably used for the upper layer, as in the case ofthe barrier film 111. Furthermore, an insulating film provided in thelower layer may be an insulating film from which oxygen is released byheating as in the case of the insulating film 214 j, so that oxygen maybe supplied also from above the semiconductor layer 101 j through thegate insulating film 102 j.

Next, the insulating film 213 j is formed (see FIG. 14C). The insulatingfilm 213 j can be formed of a stacked layer or a single layer of, forexample, silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitrideoxide, aluminum nitride, or the like. The insulating film 113 j can beformed by a sputtering method, a CVD method (including a thermal CVDmethod, an MOCVD method, a PECVD method, and the like), an MBE method,an ALD method, a PLD method, or the like, for example. In particular, itis preferable to form the film by a CVD method, further preferably aplasma CVD method because coverage can be favorable. Furthermore, athermal CVD method, an MOCVD method, or an ALD method is preferable inorder to reduce plasma damage.

Note that the insulating films 112 j, 113 j, the insulating film 212 j,and the insulating film 213 j are not shown in FIG. 6, FIGS. 7A to 7E,FIG. 8, FIG. 9, FIGS. 11A to 11C, FIG. 12, and FIGS. 18A to 18E in orderto avoid complexity.

Next, the insulating film 216 j is formed. The insulating film 216 j canbe formed of a stacked layer or a single layer of, for example, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminumnitride, or the like. The insulating film 216 j can be formed by asputtering method, a CVD method (including a thermal CVD method, anMOCVD method, a PECVD method, and the like), an MBE method, an ALDmethod, a PLD method, or the like, for example. In the case where anorganic insulating material such as an organic resin is used for theinsulating film 216 j, a coating method such as a spin coating methodmay be used for the formation. Furthermore, after the insulating film216 j is formed, planarization treatment is preferably performed on theupper surface thereof. Furthermore, the material and formation methodfor the insulating film 138 may be used for the insulating film 216 j.

Next, opening portions that reach the conductive layer 204 j_a, theconductive layer 204 j_b, and the like are provided in the insulatingfilm 216 j, the insulating film 213 j, the insulating film 212 j, thegate insulating film 202 j, and the semiconductor layer 201 j_c, aconductive film to be the plug 141 j, the plug 142 j, the plug 143 j,and the like is formed so as to fill the opening portions, andplanarization treatment is performed so that the surface of theinsulating film 216 j is exposed, whereby the plug 141 j, the plug 142j, the plug 143 j, and the like are formed (see FIG. 15A).

Next, a conductive film to be the conductive layers 151 j, 153 j, 154 j,and the like is formed over the insulating film 216 j, the plug 141 j,and the like, a mask is formed, and etching is performed to form theconductive layers 151 j, 153 j, 154 j, and the like. Then, theinsulating film 115 j is formed (see FIG. 15B). The insulating film 115j can function as an insulating film of the capacitor Ca_j. For amaterial and the like that can be used for the insulating film 115 j,the description of the gate insulating film 202 j can be referred to,for example.

Next, the conductive layer 152 j and the like are formed over theinsulating film 115 j, in a manner similar to that of the conductivelayer 151 j (see FIG. 15C). For a material and the like that can be usedfor the conductive layers 151 j, 153 j, 154 j, the conductive layer 152j, and the like, the description of the conductive layer 205 j can bereferred to, for example. In this way, the capacitor Ca_j can be formed.

Next, the insulating film 156 j is formed over the conductive layer 152j and the insulating film 115 j. The upper surface of the insulatingfilm 156 j may be planarized. Description of the insulating film 216 jis referred to for the insulating film 156 j.

Next, the transistor Ta_j is formed over the insulating film 156 j.First, the conductive layer 105 j and the like are formed over theinsulating film 156 j. The conductive layer 105 j and the like can beformed in a manner similar to that of the conductive layer 205 j. Next,the insulating film 114 j is formed (see FIG. 16A). The insulating film114 j can be formed in a manner similar to that of the insulating film214 j.

Next, opening portions are provided in the insulating film 156 j and theinsulating film 114 j, a conductive film is formed so as to fill theopening portions, and a surface of the conductive film is planarized sothat the insulating film 114 j is exposed, whereby the plug 144 j, theplug 145 j, and the like are formed.

Next, the transistor Ta_j and the like are formed (see FIG. 16B). Forthe formation of the transistor Ta_j, the description of the transistorTb_j can be referred to.

Here, as shown in FIG. 16B, the transistor Ta_j (j=1) included in thesub memory cell SCL_1 and the transistor Tb_j (j=2) included in the submemory cell SCL_2 can be formed at the same time.

Next, the capacitor Ca_j, the transistor Ta_j, and the like are formedrepeatedly in a similar manner; thus, a semiconductor element shown inFIG. 8 can be formed.

The structure described in this embodiment can be used in appropriatecombination with the structure described in the other embodiment.

Embodiment 2

In this embodiment, an oxide semiconductor which can be favorably usedfor the transistor Ta_j and the transistor Tb_j described in Embodiment1 is described. Note that for the transistor Tb_j, the description ofthe transistor Ta_j is referred to. Furthermore, for the semiconductorlayer 201 j_a, the semiconductor layer 201 j_b, and the semiconductorlayer 201 j_c, the description of the semiconductor layer 101 j_a, thesemiconductor layer 101 j_b, and the semiconductor layer 101 j_c can bereferred to, respectively. Note that it is not necessary to use the samematerial for the semiconductor layer 101 j_a and the semiconductor layer201 j_a. That is, in the case where an In-M-Zn oxide is used for thesemiconductor layer 101 j_a and the semiconductor layer 201 j_a, forexample, it is not necessary to use materials having the same atomicratio of indium to the element M and zinc. Furthermore, for example, itis not necessary to use materials having the same energy gap for thesemiconductor layer 101 j_a and the semiconductor layer 201 j_a.Furthermore, the same applies to the semiconductor layer 101 j_b and thesemiconductor layer 201 j_b, or the semiconductor layer 101 j_c and thesemiconductor layer 201 j_c.

Here, an example in which three layers, i.e., the semiconductor layer101 j_a, the semiconductor layer 101 j_b, and the semiconductor layer101 j_c are stacked and used as the oxide semiconductor as illustratedin FIG. 6 and the like is described; however, the oxide semiconductorthat can be used to the transistor Ta_j may be a single layer.Alternatively, a structure in which one or two of the semiconductorlayer 101 j_a, the semiconductor layer 101 j_b, and the semiconductorlayer 101 j_c are not provided may be employed.

The semiconductor layer 101 j_b is, for example, an oxide semiconductorcontaining indium. The semiconductor layer 101 j_b has high carriermobility (electron mobility) by containing indium, for example.Furthermore, the semiconductor layer 101 j_b preferably contains theelement M. The element M is preferably aluminum, gallium, yttrium, tin,or the like. Other elements which can be used as the element M areboron, silicon, titanium, iron, nickel, germanium, yttrium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,and the like. Note that two or more of the above elements may be used incombination as the element M. The element M is an element having a highbonding energy with oxygen, for example. The element M is an elementwhose bonding energy with oxygen is higher than that of indium, forexample. Alternatively, the element M is an element having a function ofwidening the energy gap of the oxide semiconductor, for example.Furthermore, the semiconductor layer 101 j_b preferably contains zinc.An oxide semiconductor may be crystallized easily when containing zinc.

Note that the semiconductor layer 101 j_b is not limited to the oxidesemiconductor containing indium. The semiconductor layer 101 j_b may be,for example, an oxide semiconductor which does not contain indium andcontains zinc, such as a zinc tin oxide or a gallium tin oxide, an oxidesemiconductor containing gallium, or an oxide semiconductor containingtin.

For the semiconductor layer 101 j_b, an oxide with a wide energy gap isused. The energy gap of the semiconductor layer 101 j_b is, for example,2.5 eV or larger and 4.2 eV or smaller, preferably 2.8 eV or larger and3.8 eV or smaller, more preferably 3 eV or larger and 3.5 eV or smaller.

For example, the semiconductor layer 101 j_a and the semiconductor layer101 j_c are each an oxide semiconductor which includes one or more kindsof elements other than oxygen that are included in the semiconductorlayer 101 j_b. Since the semiconductor layer 101 j_a and thesemiconductor layer 101 j_c each include one or more kinds of elementsor two or more kinds of elements other than oxygen included in thesemiconductor layer 101 j_b, an interface state is less likely to beformed at the interface between the semiconductor layer 101 j_a and thesemiconductor layer 101 j_b and the interface between the semiconductorlayer 101 j_b and the semiconductor layer 101 j_c.

The semiconductor layer 101 j_a, the semiconductor layer 101 j_b, andthe semiconductor layer 101 j_c preferably contain at least indium. Notethat in the case where the semiconductor layer 101 j_a is an In-M-Znoxide, when a summation of In and M is assumed to be 100 atomic %, it ispreferable that In be less than 50 atomic % and M be greater than 50atomic %; it is more preferable that In be less than 25 atomic % and Mbe greater than 75 atomic %, respectively. Furthermore, in the casewhere the semiconductor layer 101 j_b is an In-M-Zn oxide, when asummation of In and M is assumed to be 100 atomic %, it is preferablethat In be greater than 25 atomic % and M be less than 75 atomic %; itis more preferable that In be greater than 34 atomic % and M be lessthan 66 atomic %. Furthermore, in the case where the semiconductor layer101 j_c is an In-M-Zn oxide, when a summation of In and M is assumed tobe 100 atomic %, it is preferable that In be less than 50 atomic % and Mbe greater than 50 atomic %; it is more preferable that In be less than25 atomic % and M be greater than 75 atomic %. Note that an oxide thatis a type the same as that of the semiconductor layer 101 j_a may beused for the semiconductor layer 101 j_c.

As the semiconductor layer 101 j_b, an oxide which has higher electronaffinity than the semiconductor layer 101 j_a and the semiconductorlayer 101 j_c is used. For example, as the semiconductor layer 101 j_b,an oxide having an electron affinity higher than those of thesemiconductor layers 101 j_a and 101 j_c by 0.07 eV or higher and 1.3 eVor lower, preferably 0.1 eV or higher and 0.7 eV or lower, or morepreferably 0.15 eV or higher and 0.4 eV or lower is used. Note that theelectron affinity refers to an energy gap between the vacuum level andthe bottom of the conduction band.

Note that an indium gallium oxide has a small electron affinity and ahigh oxygen-blocking property. Therefore, the semiconductor layer 101j_c preferably contains indium gallium oxide. The gallium atomic ratio[In/(In+Ga)] is, for example, higher than or equal to 70%, preferablyhigher than or equal to 80%, further preferably higher than or equal to90%.

The semiconductor layer 101 j_c preferably contains gallium oxide. Whenthe gallium oxide is contained in the semiconductor layer 101 j_c, loweroff-state current can be obtained in some cases.

When an electric field is applied to the gate electrode of thetransistor, a channel is formed in the semiconductor layer 101 j_bhaving the highest electron affinity among the semiconductor layer 101j_a, the semiconductor layer 101 j_b, and the semiconductor layer 101j_c.

Here, a band structure is shown in FIG. 25A. A vacuum level (denoted byvacuum level), and an energy of the bottom of the conduction band(denoted by Ec) and an energy of the top of the valence band (denoted byEv) of each of the layers are shown in FIG. 25A.

Here, a mixed region of the semiconductor layer 101 j_a and thesemiconductor layer 101 j_b might exist between the semiconductor layer101 j_a and the semiconductor layer 101 j_b. Furthermore, a mixed regionof the semiconductor layer 101 j_b and the semiconductor layer 101 j_cmight exist between the semiconductor layer 101 j_b and thesemiconductor layer 101 j_c. The mixed region has a low density ofinterface states. For that reason, the stack including the semiconductorlayer 101 j_a, the semiconductor layer 101 j_b, and the semiconductorlayer 101 j_c has a band structure where energy at each interface and inthe vicinity of the interface is changed continuously (also referred toas continuous junction).

Note that FIG. 25A illustrates the case where the Ec of thesemiconductor layer 101 j_a and the second semiconductor layer 101 j_care equal to each other; however, they may be different from each other.For example, Ec of the semiconductor layer 101 j_a may be higher thanthat of the semiconductor layer 101 j_c.

At this time, electrons mainly move not in the semiconductor layer 101j_a and the semiconductor layer 101 j_c but in the semiconductor layer101 j_b (see FIG. 25B). As described above, when the interface statedensity at the interface between the semiconductor layer 101 j_a and thesemiconductor layer 101 j_b and the interface state density at theinterface between the semiconductor layer 101 j_b and the semiconductorlayer 101 j_c are lowered, the on-state current of the transistor can beincreased without interruption of the movement of electrons in thesemiconductor layer 101 j_b.

Note that in the case where the transistor has an s-channel structure,the channel is formed in the entire region of the semiconductor layer101 j_b. Therefore, as the thickness of the semiconductor layer 101 j_bis increased, the size of the channel region is increased. That is, thethicker the semiconductor layer 101 j_b is, the larger the on-statecurrent of the transistor is. For example, the semiconductor layer 101j_b may have a region with a thickness of greater than or equal to 20nm, preferably greater than or equal to 40 nm, further preferablygreater than or equal to 60 nm, and still further preferably greaterthan or equal to 100 nm. Note that there is a possibility that theproduction efficiency of the semiconductor device is decreased;therefore, for example, the semiconductor layer 101 j_b includes aregion with a thickness of, for example, less than or equal to 300 nm,preferably less than or equal to 200 nm, further preferably less than orequal to 150 nm.

Moreover, the thickness of the semiconductor layer 101 j_c is preferablyas small as possible to increase the on-state current of the transistor.For example, the semiconductor layer 101 j_c may include a region with athickness of less than 10 nm, preferably less than or equal to 5 nm,further preferably less than or equal to 3 nm Meanwhile, thesemiconductor layer 101 j_c has a function of blocking entry of elementsother than oxygen (such as hydrogen and silicon) included in theadjacent insulator into the semiconductor layer 101 j_b where a channelis formed. Thus, the semiconductor layer 101 j_c preferably has acertain thickness. For example, the semiconductor layer 101 j_c mayinclude a region with a thickness of greater than or equal to 0.3 nm,preferably greater than or equal to 1 nm, and more preferably greaterthan or equal to 2 nm. The semiconductor layer 101 j_c preferably has anoxygen blocking property to inhibit outward diffusion of oxygen releasedfrom the gate insulating film 102 j and the like.

To improve reliability, preferably, the thickness of the semiconductorlayer 101 j_a is large and the thickness of the semiconductor layer 101j_c is small. For example, the semiconductor layer 101 j_a may include aregion with a thickness of greater than or equal to 10 nm, preferablygreater than or equal to 20 nm, further preferably greater than or equalto 40 nm, and still further preferably greater than or equal to 60 nm.When the thickness of the semiconductor layer 101 j_a is made large, adistance from an interface between the adjacent insulator and thesemiconductor layer 101 j_a to the semiconductor layer 101 j_b where achannel is formed can be large. Since the production efficiency of thesemiconductor device might be decreased, the semiconductor layer 101 j_ahas a region with a thickness of, for example, less than or equal to 200nm, preferably less than or equal to 120 nm, further preferably lessthan or equal to 80 nm.

When the oxide semiconductor film contains a large amount of hydrogen,the hydrogen and an oxide semiconductor are bonded to each other, sothat part of the hydrogen serves as a donor and causes generation of anelectron which is a carrier. As a result, the threshold voltage of thetransistor shifts in the negative direction. Therefore, it is preferablethat, after formation of the oxide semiconductor film, dehydrationtreatment (dehydrogenation treatment) be performed to remove hydrogen ormoisture from the oxide semiconductor film so that the oxidesemiconductor film is highly purified to contain impurities as little aspossible.

Note that oxygen in the oxide semiconductor film is also reduced by thedehydration treatment (dehydrogenation treatment) in some cases.Therefore, it is preferable that treatment be performed so that oxygenbe added to the oxide semiconductor to fill oxygen vacancies increasedby the dehydration treatment (dehydrogenation treatment) performed onthe oxide semiconductor film. In this specification and the like,supplying oxygen to an oxide semiconductor film may be expressed asoxygen adding treatment, or treatment for making the oxygen content ofan oxide semiconductor film be in excess of that in the stoichiometriccomposition may be expressed as treatment for making an oxygen-excessstate.

In this manner, hydrogen or moisture is removed from the oxidesemiconductor film by the dehydration treatment (dehydrogenationtreatment) and oxygen vacancies therein are filled by the oxygen addingtreatment, whereby the oxide semiconductor film can be an i-type(intrinsic) oxide semiconductor film or a substantially i-type(intrinsic) oxide semiconductor film which is extremely close to ani-type. Note that “substantially intrinsic” means that the oxidesemiconductor film contains extremely few (close to zero) carriersderived from a donor and has a carrier density of lower than or equal to1×10¹⁷/cm³, lower than or equal to 1×10¹⁶/cm³, lower than or equal to1×10¹⁵/cm³, lower than or equal to 1×10¹⁴/cm³, or lower than or equal to1×10¹³/cm³.

Thus, the transistor including an i-type or substantially i-type oxidesemiconductor film can have extremely favorable off-state currentcharacteristics. For example, the drain current at the time when thetransistor including an oxide semiconductor film is in an off-state canbe less than or equal to 1×10⁻¹⁸ A, preferably less than or equal to1×10⁻²¹ A, further preferably less than or equal to 1×10⁻²⁴ A at roomtemperature (about 25° C.); or less than or equal to 1×10⁻¹⁵ A,preferably less than or equal to 1×10⁻¹⁸ A, further preferably less thanor equal to 1×10⁻²¹ A at 85° C. Note that an off state of a transistorrefers to, in the case of an n-channel type transistor, a state wherethe gate voltage is sufficiently lower than the threshold voltage.Specifically, the transistor is in an off state when the gate voltage islower than the threshold voltage by 1 V or more, 2 V or more, or 3 V ormore.

<Structure of Oxide Semiconductor>

The structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor other thanthat. Non-single-crystal oxide semiconductors include CAAC-OS (C AxisAligned Crystalline Oxide Semiconductor), a polycrystalline oxidesemiconductor, an nc-OS (nanocrystalline oxide semiconductor), anamorphous-like oxide semiconductor (a-like OS: amorphous like oxidesemiconductor), an amorphous oxide semiconductor, and the like.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductorother than that. Crystalline oxide semiconductors include a singlecrystal oxide semiconductor, a CAAC-OS, a polycrystalline oxidesemiconductor, an nc-OS, and the like.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure, for example. In other words, it can be called a structurethat has a flexible bond angle and a short-range order but does not havea long-range order.

From the opposite viewpoint, an inherently stable oxide semiconductorcannot be referred to as a completely amorphous (completely amorphous)oxide semiconductor. Moreover, an oxide semiconductor that is notisotropic (having a periodic structure in a microscopic region, forexample) cannot be referred to as a completely amorphous oxidesemiconductor. Note that an a-like OS has a periodic structure in amicroscopic region, but at the same time has a void (also referred to asa void) and has an unstable structure. For this reason, it can be saidthat it is close to an amorphous oxide semiconductor in terms of aphysical property.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

When a combined analysis image (also referred to as a high-resolutionTEM image) of a bright-field image and a diffraction pattern of aCAAC-OS is observed with a transmission electron microscope (TEM:Transmission Electron Microscope), a plurality of pellets can beobserved. However, in the high-resolution TEM image, a boundary betweenpellets, that is, a grain boundary (also referred to as a grainboundary) cannot be clearly observed. Thus, in the CAAC-OS, a reductionin electron mobility due to the grain boundary is less likely to occur.

The CAAC-OS observed with a TEM is described below. FIG. 20A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.A spherical aberration corrector (Spherical Aberration Corrector)function was used for the observation of the high-resolution TEM image.The high-resolution TEM image using a spherical aberration correctorfunction is particularly referred to as a Cs-corrected high-resolutionTEM image. The Cs-corrected high-resolution TEM image can be obtainedwith, for example, an atomic resolution analytical electron microscopeJEM-ARM200F manufactured by JEOL Ltd.

FIG. 20B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 20A. It can be confirmed from FIG. 20B that metalatoms are arranged in a layered manner in a pellet. Each metal atomlayer reflects unevenness of a surface over which a film of the CAAC-OSis formed (hereinafter, the surface is referred to as a formationsurface) or a top surface thereof, and is arranged parallel to theformation surface or the top surface of the CAAC-OS.

As shown in FIG. 20B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 20C. FIG. 20B and FIG. 20C prove that the size ofa pellet is greater than or equal to 1 nm or greater than or equal to 3nm, and the size of a space caused by tilt of the pellets isapproximately 0.8 nm. Therefore, the pellet can also be referred to as ananocrystal (nc: nanocrystal). Furthermore, a CAAC-OS can be referred toas an oxide semiconductor including CANC (C-Axis Aligned Nanocrystals).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is schematically illustrated by such a structure in which bricks orblocks are stacked (see FIG. 20D). The part in which the pellets aretilted as observed in FIG. 20C corresponds to a region 5161 shown inFIG. 20D.

FIG. 21A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIG. 21B, FIG. 21C, and FIG. 21D are enlargedCs-corrected high-resolution TEM images of a region (1), a region (2),and a region (3) in FIG. 21A, respectively. It can be confirmed fromFIG. 21B, FIG. 21C, and FIG. 21D that metal atoms are arranged in atriangular, quadrangular, or hexagonal configuration in a pellet.However, there is no regularity of arrangement of metal atoms betweendifferent pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD: X-Ray Diffraction)is described. For example, when the structure of a CAAC-OS including anInGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appearsat a diffraction angle (2θ) of around 31° as shown in FIG. 22A. Thispeak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS have c-axis alignment, and thatthe c-axes are aligned in a direction substantially perpendicular to theformation surface or the top surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak of 2θ at around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis by an in-plane method in whichan X-ray is incident on the CAAC-OS in a direction substantiallyperpendicular to its c-axis, a peak appears when 2θ is around 56°. Thispeak is derived from the (110) plane of the InGaZnO₄ crystal. In thecase of the CAAC-OS, when analysis (φ scan) is performed with 2θ fixedat around 56° and with the sample rotated using a normal vector of thesample surface as an axis (φ axis), as shown in FIG. 22B, a clear peakis not observed. In contrast, in the case of a single crystal oxidesemiconductor of InGaZnO₄, when φ scan is performed with 2θ fixed ataround 56°, as shown in FIG. 22C, six peaks which are derived fromcrystal planes equivalent to the (110) plane are observed. Accordingly,the structural analysis using XRD shows that the directions of a-axesand b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) as shownin FIG. 23A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 23B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 23B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. Note that the first ring in FIG. 23B is considered to bederived from the (010) plane, the (100) plane, and the like of theInGaZnO₄ crystal. Furthermore, the second ring in FIG. 23B is consideredto be derived from the (110) plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. From theopposite viewpoint, the CAAC-OS has small amounts of impurities anddefects (e.g., oxygen vacancies).

Note that the impurity is an element other than the main components ofthe oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element, such as silicon, thathas higher strength of bonding to oxygen than a metal element includedin an oxide semiconductor extracts oxygen from the oxide semiconductor,which results in disorder of the atomic arrangement and reducedcrystallinity of the oxide semiconductor. A heavy metal such as iron ornickel, argon, carbon dioxide, or the like has a large atomic radius (ormolecular radius), and thus disturbs the atomic arrangement of the oxidesemiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiescontained in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. Furthermore, oxygen vacanciesin the oxide semiconductor serve as carrier traps or serve as carriergeneration sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor with low carrier density. Specifically, an oxidesemiconductor with a carrier density of lower than 8×10¹¹/cm³,preferably lower than 1×10¹¹/cm³, more preferably lower than 1×10¹⁰/cm³,and higher than or equal to 1×10⁻⁹/cm³ can be used. Such an oxidesemiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

<nc-OS>

Next, an nc-OS is described.

An nc-OS has a region in which a crystal part is observed and a regionin which a clear crystal part is not observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part with a size greater than10 nm and less than or equal to 100 nm is referred to as amicrocrystalline oxide semiconductor in some cases. In a high-resolutionTEM image of the nc-OS, for example, a grain boundary is not clearlyobserved in some cases. Note that there is a possibility that the originof the nanocrystal is the same as that of a pellet in a CAAC-OS.Therefore, a crystal part of the nc-OS may be referred to as a pellet inthe following description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is subjected to analysis by an out-of-planemethod using an X-ray having a diameter larger than the size of apellet, a peak which shows a crystal plane does not appear. Furthermore,a diffraction pattern like a halo pattern is observed when the nc-OS issubjected to electron diffraction using an electron beam with a probediameter (e.g., 50 nm or larger) that is larger than the size of apellet. Meanwhile, spots appear in a nanobeam electron diffractionpattern of the nc-OS when an electron beam having a probe diameter closeto or smaller than the size of a pellet is applied. Moreover, in ananobeam electron diffraction pattern of the nc-OS, regions with highluminance in a circular (ring) pattern are shown in some cases.Moreover, a plurality of spots are shown in a ring-like region in somecases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including RANC (Random Aligned nanocrystals) oran oxide semiconductor including NANC (Non-Aligned nanocrystals).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS or an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-Like OS>

An a-like OS is an oxide semiconductor having a structure intermediatebetween the nc-OS and the amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (referred to as sample A), an nc-OS (referred to as sampleB), and a CAAC-OS (referred to as sample C) are prepared as samplessubjected to electron irradiation. Each of the samples is an In—Ga—Znoxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of the InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. Accordingly, thedistance between the adjacent layers is equivalent to the latticespacing on the (009) plane (also referred to as d value), and the valueis calculated to be 0.29 nm from crystal structural analysis.Accordingly, a portion where the spacing between lattice fringes isgreater than or equal to 0.28 nm and less than or equal to 0.30 nm isregarded as a crystal part of InGaZnO₄. Each of lattice fringescorresponds to the a-b plane of the InGaZnO₄ crystal.

FIG. 24 shows the average size of crystal parts (at 22 points to 45points) in each sample. Note that the crystal part size corresponds tothe length of a lattice fringe. FIG. 24 indicates that the crystal partsize in the a-like OS increases with the cumulative electron dose.Specifically, as shown by (1) in FIG. 24, a crystal part ofapproximately 1.2 nm (also referred to as an initial nucleus) at thestart of TEM observation grows to a size of approximately 2.6 nm at acumulative electron dose of 4.2×10⁸ e⁻/nm². In contrast, the crystalpart size in the nc-OS and the CAAC-OS shows little change from thestart of electron irradiation to a cumulative electron dose of 4.2×10⁸e⁻/nm². Specifically, as shown by (2) and (3) in FIG. 24, the averagecrystal sizes in an nc-OS and a CAAC-OS are approximately 1.4 nm andapproximately 2.1 nm, respectively, regardless of the cumulativeelectron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

Furthermore, the a-like OS has a lower density than the nc-OS and theCAAC-OS because it includes a void. Specifically, the density of thea-like OS is higher than or equal to 78.6% and lower than 92.3% of thedensity of a single crystal having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of a single crystal having the samecomposition. It is difficult to deposit an oxide semiconductor having adensity of lower than 78% of the density of the single crystal.

For example, in the case of an oxide semiconductor satisfyingIn:Ga:Zn=1:1:1 [atomic ratio], the density of single crystal InGaZnO₄with a rhombohedral crystal structure is 6.357 g/cm³. Accordingly, forexample, in the case of the oxide semiconductor satisfyingIn:Ga:Zn=1:1:1 [atomic ratio], the density of the a-like OS is higherthan or equal to 5.0 g/cm³ and lower than 5.9 g/cm³. Furthermore, forexample, in the case of the oxide semiconductor satisfyingIn:Ga:Zn=1:1:1 [atomic ratio], the density of each of the nc-OS and theCAAC-OS is higher than or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that single crystals with the same composition do not exist in somecases. In that case, single crystals with different compositions arecombined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal with the desiredcomposition. The density of a single crystal having the desiredcomposition can be calculated using a weighted average according to thecombination ratio of the single crystals with different compositions.Note that it is preferable to use as few kinds of single crystals aspossible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedfilm including two or more of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

The case where an oxide semiconductor contains indium, an element M, andzinc is considered. Here, the element M is preferably aluminum, gallium,yttrium, tin, or the like. Other elements which can be used as theelement M are boron, silicon, titanium, iron, nickel, germanium,yttrium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium,tantalum, tungsten, and the like. Note that two or more of the aboveelements may be used in combination as the element M. A favorable rangeof the atomic ratio of indium to the element M and zinc, x:y:z, of theoxide semiconductor is described.

It is known that there is a homologous series (homologous series)represented by InMO₃(ZnO)_(m) (m is a natural number) as an oxidecontaining indium, the element M, and zinc. Here, for example, the casewhere the element M is Ga is considered.

For example, a compound represented by ZnM₂O₄, such as ZnGa₂O₄, is knownas a compound having a spinel crystal structure, for example.Furthermore, for example, when a composition is in the neighborhood ofZnGa₂O₄, that is, values of x, y and z are close to (x,y,z)=(0, 1, 2), aspinel crystal structure is likely to be formed or mixed. Here, theoxide semiconductor is preferably a CAAC-OS. Furthermore, it ispreferable that the CAAC-OS have no spinel crystal structure inparticular. In addition, to increase carrier mobility, the percentage ofIn content is preferably increased. In an oxide semiconductor containingindium, the element M, and zinc, the s orbital of heavy metal mainlycontributes to carrier transfer, and when the percentage of indiumcontent in the oxide semiconductor is increased, overlaps of the sorbitals are increased; therefore, the mobility of oxide in which thepercentage of indium content is high is higher than that of oxide inwhich the percentage of indium content is low. Therefore, when oxidehaving a high indium content is used as the oxide semiconductor, carriermobility can be increased.

For example, to increase carrier mobility, it is preferable to increasethe proportion of indium atoms. For example, in the case where theatomic ratio of indium, the element M, and zinc contained in the oxidesemiconductor is represented by x:y:z, it is preferable that x be 1.75or more times as large as y.

Furthermore, to further increase the CAAC proportion of the oxidesemiconductor, the proportion of zinc atoms is preferably increased. Forexample, when the atomic ratio of an In—Ga—Zn oxide is within the rangewhich allows a solid solution range, the CAAC proportion can be furtherincreased. The range which allows a solid solution range tends to bewidened when the ratio of the number of zinc atoms to the total numberof indium and gallium atoms is increased. Therefore, when the ratio ofthe number of zinc atoms to the total number of indium and gallium atomsis increased, the CAAC proportion of the oxide semiconductor can befurther increased in some cases. For example, in the case where theatomic ratio of indium, the element M, and zinc contained in the oxidesemiconductor is represented by x:y:z, it is preferable that z be 0.5 ormore times as large as x+y. On the other hand, to increase the atomicratio of indium and increase carrier mobility, it is preferable that zbe two or less times as large as x+y.

Consequently, a spinel crystal structure is hardly observed or is notobserved by nanobeam electron diffraction. Thus, an excellent CAAC-OScan be obtained. Furthermore, carrier scattering or the like at theboundary between a CAAC structure and a spinel crystal structure can bereduced; therefore, when the oxide semiconductor is used for atransistor, a transistor having high field-effect mobility can beobtained. In addition, a transistor having high reliability can beobtained.

As a result, an oxide semiconductor having a high CAAC proportion can beobtained. That is, a high-quality CAAC-OS can be obtained. Furthermore,a CAAC-OS having no region or a very few regions in which a spinelcrystal structure is observed can be obtained. For example, ahigh-quality CAAC-OS has a proportion of CAAC of higher than or equal to50%, preferably higher than or equal to 80%, more preferably higher thanor equal to 90%, still more preferably higher than or equal to 95% andlower than or equal to 100%.

Furthermore, in the case where a film of an oxide semiconductor isformed by a sputtering method, a film having an atomic ratio differentfrom that of a target may be formed. Especially for zinc, the atomicratio of zinc in a film is smaller than the atomic ratio of the targetin some cases. Specifically, the film has an atomic ratio of zinc of 40atomic % to approximately 90 atomic % of the atomic ratio of zinc in thetarget in some cases.

Therefore, the atomic ratio of zinc in the target is preferably higherthan the atomic ratio of zinc in the oxide semiconductor obtained by asputtering method.

Note that a plurality of films may be stacked in the oxidesemiconductor. Furthermore, the plurality of films may differ in theCAAC proportion. In addition, the CAAC proportion of at least one of thestacked films is, for example, 90% higher, further preferably 95% orhigher, still further preferably 97% or higher and 100% or lower.

A CAAC-OS film can be formed, for example, by the following method.

A CAAC-OS film is formed, for example, by a sputtering method using apolycrystalline oxide semiconductor sputtering target. As a sputteringmethod, an RF sputtering method, a DC sputtering method, an ACsputtering method, or the like can be used. To improve uniformity offilm thickness distribution of the oxide semiconductor film, filmcomposition distribution, and crystallinity distribution, a DCsputtering method or an AC sputtering method is preferably used ratherthan an RF sputtering method.

By increasing the substrate temperature during deposition, migration ofsputtered particles is likely to occur after the sputtered particlesreach a substrate surface. Specifically, the substrate temperatureduring the deposition is higher than or equal to 100° C. and lower thanor equal to 740° C., preferably higher than or equal to 200° C. andlower than or equal to 500° C. By increasing the substrate temperatureduring the deposition, when sputtered particles reach the substrate,migration occurs on the substrate, so that a flat plane of the sputteredparticles is attached to the substrate. At this time, the sputteredparticle is charged positively, whereby sputtered particles are attachedto the substrate while repelling each other; thus, the sputteredparticles do not overlap with each other randomly, and a CAAC-OS filmwith a uniform thickness can be deposited.

By reducing the entry of impurities during the deposition, the crystalstate can be prevented from being broken by the impurities. For example,the concentration of impurities (hydrogen, water, carbon dioxide,nitrogen, and the like) which exist in a deposition chamber may bereduced. Furthermore, the concentration of impurities in a depositiongas may be reduced. Specifically, a deposition gas whose dew point is−80° C. or lower, preferably −100° C. or lower is used.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

Alternatively, a CAAC-OS film is formed by the following method.

First, a first oxide semiconductor film is formed to a thickness ofgreater than or equal to 1 nm and less than 10 nm. The first oxidesemiconductor film is formed by a sputtering method. Specifically, thesubstrate temperature during the deposition is set to higher than orequal to 100° C. and lower than or equal to 500° C., preferably higherthan or equal to 150° C. and lower than or equal to 450° C., and theproportion of oxygen in a deposition gas is set to higher than or equalto 30 vol %, preferably 100 vol %.

Next, heat treatment is performed so that the first oxide semiconductorfilm becomes a first CAAC-OS film with high crystallinity. Thetemperature of the heat treatment is higher than or equal to 350° C. andlower than or equal to 740° C., preferably higher than or equal to 450°C. and lower than or equal to 650° C. The heat treatment time is longerthan or equal to 1 minute and shorter than or equal to 24 hours,preferably longer than or equal to 6 minutes and shorter than or equalto 4 hours. The heat treatment may be performed in an inert atmosphereor an oxidation atmosphere. It is preferable to perform heat treatmentin an inert atmosphere and then perform heat treatment in an oxidationatmosphere. The heat treatment in an inert atmosphere can reduce theconcentration of impurities in the first oxide semiconductor film for ashort time. At the same time, the heat treatment in an inert atmospheremay generate oxygen vacancies in the first oxide semiconductor film. Insuch a case, the heat treatment in an oxidation atmosphere can reducethe oxygen vacancies. Note that the heat treatment may be performedunder a reduced pressure, such as 1000 Pa or lower, 100 Pa or lower, 10Pa or lower, or 1 Pa or lower. Under the reduced pressure, theconcentration of impurities in the first oxide semiconductor film can bereduced for a shorter time.

The first oxide semiconductor film can be crystallized easier in thecase where the thickness is greater than or equal to 1 nm and less than10 nm than in the case where the thickness is greater than or equal to10 nm.

Next, a second oxide semiconductor film having the same composition asthe first oxide semiconductor film is formed to a thickness of greaterthan or equal to 10 nm and less than or equal to 50 nm. The second oxidesemiconductor film is formed by a sputtering method. Specifically, thesubstrate temperature during the deposition is set to higher than orequal to 100° C. and lower than or equal to 500° C., preferably higherthan or equal to 150° C. and lower than or equal to 450° C., and theproportion of oxygen in a deposition gas is set to higher than or equalto 30 vol %, preferably 100 vol %.

Next, heat treatment is performed so that solid phase growth of thesecond oxide semiconductor film is performed using the first CAAC-OSfilm, thereby forming a second CAAC-OS film with high crystallinity. Thetemperature of the heat treatment is higher than or equal to 350° C. andlower than or equal to 740° C., preferably higher than or equal to 450°C. and lower than or equal to 650° C. The heat treatment time is longerthan or equal to 1 minute and shorter than or equal to 24 hours,preferably longer than or equal to 6 minutes and shorter than or equalto 4 hours. The heat treatment may be performed in an inert atmosphereor an oxidation atmosphere. It is preferable to perform heat treatmentin an inert atmosphere and then perform heat treatment in an oxidationatmosphere. The heat treatment in an inert atmosphere can reduce theconcentration of impurities in the second oxide semiconductor film for ashort time. At the same time, the heat treatment in an inert atmospheremay generate oxygen vacancies in the second oxide semiconductor film. Insuch a case, the heat treatment in an oxidation atmosphere can reducethe oxygen vacancies. Note that the heat treatment may be performedunder a reduced pressure, such as 1000 Pa or lower, 100 Pa or lower, 10Pa or lower, or 1 Pa or lower. Under the reduced pressure, theconcentration of impurities in the second oxide semiconductor film canbe reduced for a shorter time.

As described above, a CAAC-OS film with a total thickness of greaterthan or equal to 10 nm can be formed.

At least part of this embodiment can be implemented in combination withthe other embodiment described in this specification as appropriate.

Embodiment 3

In this embodiment, an RF tag including a memory device such as thememory cell array 300 illustrated in the above embodiment is describedwith reference to FIG. 27. Here, the memory device may include astructure including a row selection driver, a column selection driver,an A/D converter, and the like that are connected to the memory cellarray.

The RF tag of this embodiment includes a memory circuit, storesnecessary data in the memory circuit, and transmits and receives datato/from the outside by using contactless means, for example, wirelesscommunication. With these features, the RF tag can be used for anindividual authentication system or the like in which the individualinformation of an object or the like is read to recognize the object.Note that extremely high reliability is required for these applications.

A configuration of the RF tag is described with reference to FIG. 27.FIG. 27 is a block diagram illustrating a configuration example of an RFtag.

As shown in FIG. 27, an RF tag 800 includes an antenna 804 that receivesa radio signal 803 that is transmitted from an antenna 802 connected toa communication device 801 (also referred to as an interrogator, areader/writer, or the like). The RF tag 800 further includes a rectifiercircuit 805, a constant voltage circuit 806, a demodulation circuit 807,a modulation circuit 808, a logic circuit 809, a memory circuit 810, anda ROM 811. Note that a material that enables a reverse current to be lowenough, for example, an oxide semiconductor may be used for a transistorhaving a rectifying function included in the demodulation circuit 807.This can suppress the phenomenon of a rectifying function becomingweaker due to a reverse current and prevent saturation of the outputfrom the demodulation circuit. In other words, the input to thedemodulation circuit and the output from the demodulation circuit canhave a relation closer to a linear relation. Note that data transmissionmethods are roughly classified into the following three methods: anelectromagnetic coupling method in which a pair of coils is provided soas to face each other and communication is performed by mutualinduction, an electromagnetic induction method in which communication isperformed using an induction field, and a radio wave method in whichcommunication is performed using a radio wave. Any of these methods canbe used in the RF tag 800 described in this embodiment.

Next, a configuration of each circuit is described. The antenna 804 isan object for exchanging the radio signal 803 with the antenna 802 thatis connected to the communication device 801. Furthermore, the rectifiercircuit 805 is a circuit for generating an input potential byrectification, for example, half-wave voltage doubler rectification ofan input alternating signal generated by reception of a radio signal atthe antenna 804 and smoothing of the rectified signal with a capacitorprovided in a later stage. Note that a limiter circuit may be providedon an input side or an output side of the rectifier circuit 805. Thelimiter circuit is a circuit for controlling electric power so thatelectric power that is higher than or equal to certain electric power isnot input to a circuit in a later stage if the amplitude of the inputalternating signal is high and an internal generation voltage is high.

The constant voltage circuit 806 is a circuit for generating a stablepower supply voltage from an input potential and supplies it to eachcircuit. Note that the constant voltage circuit 806 may include a resetsignal generation circuit. The reset signal generation circuit is acircuit for generating a reset signal of the logic circuit 809 byutilizing rise of the stable power supply voltage.

The demodulation circuit 807 is a circuit for demodulating the inputalternating signal by envelope detection to generate the demodulatedsignal. Furthermore, the modulation circuit 808 is a circuit forperforming modulation in accordance with data to be output from theantenna 804.

The logic circuit 809 is a circuit for analyzing and processing thedemodulated signal. The memory circuit 810 is a circuit for holding theinput data and includes a row decoder, a column decoder, a memoryregion, and the like. Furthermore, the ROM 811 stores an identificationnumber (ID) or the like and outputs it in accordance with processing.

Note that the above-described circuits may be selected as appropriate asneeded.

Here, the memory circuit described in the above embodiment can be usedas the memory circuit 810. Since the memory circuit of one embodiment ofthe present invention can retain data even when not powered, it can befavorably used for an RF tag. In addition, the memory circuit of oneembodiment of the present invention needs power (voltage) needed fordata writing significantly lower than that needed in a conventionalnonvolatile memory; thus, it is possible to prevent a difference betweenthe maximum communication range in data reading and that in datawriting. Furthermore, it is possible to suppress malfunction orincorrect writing that is caused by power shortage in data writing.

Furthermore, since the memory circuit of one embodiment of the presentinvention can be used as a nonvolatile memory, it can also be used asthe ROM 811. In this case, it is preferable that a manufacturerseparately prepare a command for writing data to the ROM 811 so that auser cannot rewrite data freely. Since the manufacturer writesidentification numbers before shipment and then starts shipment ofproducts, instead of putting identification numbers to all themanufactured RF tags, it is possible to put identification numbers onlyto good products to be shipped. Thus, the identification numbers of theshipped products are in series and customer management corresponding tothe shipped products is easily performed.

At least part of this embodiment can be implemented in combination withthe other embodiment described in this specification as appropriate.

Embodiment 4

In this embodiment, a CPU including a memory device including at leastthe memory cell array 300 described in the embodiment is described.Here, the memory device may include a structure including a rowselection driver, a column selection driver, an A/D converter, and thelike that are connected to the memory cell array.

FIG. 28 is a block diagram illustrating a configuration of an example ofa CPU at least partly including the memory device described in theaforementioned embodiment.

The CPU illustrated in FIG. 28 includes, over a substrate 1190, an ALU1191 (ALU: Arithmetic Logic Unit, arithmetic circuit), an ALU controller1192, an instruction decoder 1193, an interrupt controller 1194, atiming controller 1195, a register 1196, a register controller 1197, abus interface 1198 (Bus I/F), a rewritable ROM 1199, and a ROM interface1189 (ROM I/F). A semiconductor substrate, an SOI substrate, a glasssubstrate, or the like is used as the substrate 1190. The ROM 1199 andthe ROM interface 1189 may be provided over a separate chip. Needless tosay, the CPU in FIG. 28 is just an example in which the configuration issimplified, and an actual CPU has a variety of configurations dependingon the application. For example, the CPU may have a configurationincluding a plurality of cores that operate in parallel; each of thecores has a structure including the CPU or the arithmetic circuitillustrated in FIG. 28. Furthermore, the number of bits that the CPU canprocess in an internal arithmetic circuit or in a data bus can be, forexample, 8, 16, 32, or 64.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 processes an interrupt request from an external input/output deviceor a peripheral circuit depending on its priority or a mask state. Theregister controller 1197 generates an address of the register 1196, andreads/writes data from/to the register 1196 depending on the state ofthe CPU.

Furthermore, the timing controller 1195 generates signals forcontrolling operation timings of the ALU 1191, the ALU controller 1192,the instruction decoder 1193, the interrupt controller 1194, and theregister controller 1197. For example, the timing controller 1195includes an internal clock generator for generating an internal clocksignal CLK2 on the basis of a reference clock signal CLK1, and suppliesthe internal clock signal CLK2 to the above circuits.

In the CPU illustrated in FIG. 28, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, the transistordescribed in the aforementioned embodiment can be used.

In the CPU illustrated in FIG. 28, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, whether data is retained by aflip-flop or by a capacitor in the memory cell included in the register1196 is selected. When data retaining by the flip-flop is selected, apower supply voltage is supplied to the memory cell in the register1196. When data retaining by the capacitor is selected, the data isrewritten in the capacitor, and supply of a power supply voltage to thememory cell in the register 1196 can be stopped.

FIG. 29 is an example of a circuit diagram of a memory element that canbe used for the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, a switch 1203, a switch 1204, a logic element 1206, acapacitor 1207, and a circuit 1220 having a selecting function. Thecircuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.The transistor 1209 is preferably a transistor in which a channel isformed in an oxide semiconductor layer.

Here, the memory device described in the aforementioned embodiment canbe used as the circuit 1202. When supply of a power supply voltage tothe memory element 1200 is stopped, a ground potential (0 V) or apotential at which the transistor 1209 in the circuit 1202 is turned offcontinues to be input to a gate of the transistor 1209. For example, thegate of the transistor 1209 is grounded through a load such as aresistor.

Shown here is an example in which the switch 1203 is composed of atransistor 1213 having one conductivity type (e.g., an n-channel type)and the switch 1204 is composed of a transistor 1214 having aconductivity type opposite to the one conductivity type (e.g., ap-channel type). Here, a first terminal of the switch 1203 correspondsto one of a source and a drain of the transistor 1213, a second terminalof the switch 1203 corresponds to the other of the source and the drainof the transistor 1213, and conduction or non-conduction between thefirst terminal and the second terminal of the switch 1203 (i.e., the onstate or the off state of the transistor 1213) is selected by a controlsignal RD input to a gate of the transistor 1213. A first terminal ofthe switch 1204 corresponds to one of a source and a drain of thetransistor 1214, a second terminal of the switch 1204 corresponds to theother of the source and the drain of the transistor 1214, and conductionor non-conduction between the first terminal and the second terminal ofthe switch 1204 (i.e., the on state or the off state of the transistor1214) is selected by the control signal RD input to a gate of thetransistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a wiring that can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a wiring that cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. To the other of the pair of electrodes of the capacitor 1207,a constant potential can be input. For example, a low power supplypotential (e.g., GND) or a high power supply potential (e.g., VDD) canbe input. The other of the pair of electrodes of the capacitor 1207 iselectrically connected to the wiring that can supply a low power supplypotential (e.g., a GND line). To the other of the pair of electrodes ofthe capacitor 1208, a constant potential can be input. For example, alow power supply potential (e.g., GND) or a high power supply potential(e.g., VDD) can be input. The other of the pair of electrodes of thecapacitor 1208 is electrically connected to the wiring that can supply alow power supply potential (e.g., a GND line).

Note that the capacitor 1207 and the capacitor 1208 can be omitted aslong as the parasitic capacitance of the transistor, the wiring, or thelike is actively utilized.

A control signal WE is input to the first gate (first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD that isdifferent from the control signal WE; when the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 29illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example illustrated in FIG. 29, a signal output from the secondterminal of the switch 1203 (the other of the source and the drain ofthe transistor 1213) is input to the circuit 1201 through the logicelement 1206 and the circuit 1220; however, it is not limited thereto.The signal output from the second terminal of the switch 1203 (the otherof the source and the drain of the transistor 1213) may be input to thecircuit 1201 without its logic value being inverted. For example, in thecase where the circuit 1201 includes a node in which a signal obtainedby inversion of the logic value of a signal input from the inputterminal is retained, the signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) can be input to the node.

Furthermore, in FIG. 29, the transistors included in the memory element1200 except for the transistor 1209 can each be a transistor in which achannel is formed in a layer formed using a semiconductor other than anoxide semiconductor or in the substrate 1190. For example, thetransistor can be a transistor whose channel is formed in a siliconlayer or a silicon substrate. Alternatively, a transistor in which achannel is formed in an oxide semiconductor layer can be used for allthe transistors in the memory element 1200. Further alternatively, inthe memory element 1200, a transistor in which a channel is formed in anoxide semiconductor layer can be included besides the transistor 1209,and a transistor in which a channel is formed in a layer or thesubstrate 1190 including a semiconductor other than an oxidesemiconductor can be used for the rest of the transistors.

As the circuit 1201 in FIG. 29, for example, a flip-flop circuit can beused. Furthermore, as the logic element 1206, for example, an inverter,a clocked inverter, or the like can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 that is provided in the circuit 1202.

Furthermore, the off-state current of a transistor in which a channel isformed in an oxide semiconductor layer is extremely small. For example,the off-state current of a transistor in which a channel is formed in anoxide semiconductor layer is significantly smaller than that of atransistor in which a channel is formed in silicon having crystallinity.Thus, when the transistor is used as the transistor 1209, a signalretained in the capacitor 1208 is retained for a long time also in aperiod during which the power supply voltage is not supplied to thememory element 1200. The memory element 1200 can accordingly retain thestored content (data) also in a period during which the supply of thepower supply voltage is stopped.

Furthermore, since the memory element is characterized by performingpre-charge operation with the switch 1203 and the switch 1204, the timerequired for the circuit 1201 to retain original data again after thesupply of the power supply voltage is restarted can be shortened.

Furthermore, in the circuit 1202, a signal retained by the capacitor1208 is input to the gate of the transistor 1210. Thus, after supply ofthe power supply voltage to the memory element 1200 is restarted, thesignal retained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 changes tosome degree.

By using the above-described memory element 1200 in a memory device suchas a register or a cache memory included in a processor, data in thememory device can be prevented from being lost owing to the stop of thesupply of the power supply voltage. Furthermore, return to the samestate as that before the power supply is stopped is possible shortlyafter the supply of the power supply voltage is restarted. Thus, thepower supply can be stopped even for a short time in the entireprocessor or one or a plurality of logic circuits included in theprocessor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU in this embodiment,the memory element 1200 can also be used in an LSI such as a DSP(Digital Signal Processor), a custom LSI, or a PLD (Programmable LogicDevice), and an RF-ID (Radio Frequency Identification).

Note that in this specification and the like, a transistor can be formedusing a variety of substrates, for example. The type of a substrate isnot limited to a certain type. As an example of the substrate, asemiconductor substrate (e.g., a single crystal substrate or a siliconsubstrate), an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a metal substrate, a stainless steel substrate, asubstrate including stainless steel foil, a tungsten substrate, asubstrate including tungsten foil, a flexible substrate, an attachmentfilm, paper including a fibrous material, a base material film, or thelike is given. As an example of a glass substrate is barium borosilicateglass, aluminoborosilicate glass, soda lime glass, or the like is given.As an example of a flexible substrate, an attachment film, a basematerial film, or the like, the following is given. For example, plastictypified by polyethylene terephthalate (PET), polyethylene naphthalate(PEN), or polyether sulfone (PES) is given. Another example is asynthetic resin such as acrylic, or the like. Another example ispolypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, or thelike. Another example is polyamide, polyimide, aramid, epoxy, aninorganic vapor deposition film, paper, or the like. Specifically, theuse of semiconductor substrates, single crystal substrates, SOIsubstrates, or the like enables the manufacture of small-sizedtransistors with a small variation in characteristics, size, shape, orthe like and with high current capability. A circuit using suchtransistors achieves lower power consumption of the circuit or higherintegration of the circuit.

Alternatively, a flexible substrate may be used as the substrate, andthe transistor may be provided directly on the flexible substrate.Further alternatively, a separation layer may be provided between thesubstrate and the transistor. The separation layer can be used when partor the whole of a semiconductor device formed over the separation layeris separated from the substrate and transferred onto another substrate.In such a case, the transistor can be transferred to a substrate havinglow heat resistance or a flexible substrate as well. Note that for theabove separation layer, a structure of a stacked-layer structure ofinorganic films, which are a tungsten film and a silicon oxide film, astructure in which an organic resin film of polyimide or the like isformed over a substrate, or the like can be used, for example.

In other words, a transistor may be formed using one substrate, and thentransferred to another substrate, so that the transistor is providedover the another substrate. As an example of a substrate to which atransistor is transferred, in addition to the above-described substrateover which the transistor can be formed, a paper substrate, a cellophanesubstrate, an aramid film substrate, a polyimide film substrate, a stonesubstrate, a wood substrate, a cloth substrate (including a naturalfiber (silk, cotton, hemp), a synthetic fiber (nylon, polyurethane,polyester), a regenerated fiber (acetate, cupra, rayon, regeneratedpolyester), or the like), a leather substrate, a rubber substrate, orthe like. When such a substrate is used, a transistor with excellentproperties or a transistor with low power consumption can be formed, adevice with high durability can be manufactured, high heat resistancecan be provided, or reduction in weight or thickness can be achieved.

For example, in this specification and the like, when it is explicitlydescribed that X and Y are connected, the case where X and Y areelectrically connected, the case where X and Y are functionallyconnected, and the case where X and Y are directly connected areincluded therein. Accordingly, a connection relationship other thanthose shown in drawings and texts is also included without limitation toa predetermined connection relationship, for example, the connectionrelationship shown in the drawings and the texts.

Here, X and Y each denote an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, alayer).

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, a load) can beconnected between X and Y. Note that a switch has a function of beingcontrolled to be on or off. That is, a switch has a function ofdetermining whether current flows or not by being turned on (on state)or turned off (off state). Alternatively, the switch has a function ofselecting and changing a current path.

As an example of the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y (forexample, a logic circuit (e.g., an inverter, a NAND circuit, or a NORcircuit), a signal converter circuit (e.g., a DA converter circuit, anAD converter circuit, or a gamma correction circuit), a potential levelconverter circuit (e.g., a power supply circuit (a step-up circuit, astep-down circuit, or the like), a level shifter circuit for changingthe potential level of a signal), a voltage source, a current source, aswitching circuit, an amplifier circuit (e.g., a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, a buffer circuit), a signal generation circuit, amemory circuit, or a control circuit) can be connected between X and Y.Note that for example, in the case where a signal output from X istransmitted to Y even when another circuit is interposed between X andY, X and Y are functionally connected.

Note that when it is explicitly described that X and Y are connected,the case where X and Y are electrically connected (i.e., the case whereX and Y are connected with another element or another circuit providedtherebetween), the case where X and Y are functionally connected (i.e.,the case where X and Y are functionally connected with another circuitprovided therebetween), and the case where X and Y are directlyconnected (i.e., the case where X and Y are connected without anotherelement or another circuit provided therebetween) are included therein.That is, when it is explicitly described that X and Y are electricallyconnected, the description is the same as the case where it isexplicitly only described that X and Y are connected.

Note that, for example, the case where a source (or a first terminal orthe like) of a transistor is electrically connected to X through (or notthrough) Z1 and a drain (or a second terminal or the like) of thetransistor is electrically connected to Y through (or not through) Z2,or the case where a source (or a first terminal or the like) of atransistor is directly connected to one part of Z1 and another part ofZ1 is directly connected to X while a drain (or a second terminal or thelike) of the transistor is directly connected to one part of Z2 andanother part of Z2 is directly connected to Y, can be expressed by usingthe following expressions.

For example, the expression “X, Y, and a source (or a first terminal orthe like) and a drain (or a second terminal or the like) of a transistorare electrically connected to each other, and X, the source (or thefirst terminal or the like) of the transistor, the drain (or the secondterminal or the like) of the transistor, and Y are electricallyconnected to each other in this order”, the expression “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, or the expression “X is electrically connected to Ythrough a source (or a first terminal or the like) and a drain (or asecond terminal or the like) of a transistor, and X, the source (or thefirst terminal or the like) of the transistor, the drain (or the secondterminal or the like) of the transistor, and Y are provided to beconnected in this order” can be employed. When the connection order in acircuit configuration is defined by an expression similar to theseexamples, a source (or a first terminal or the like) and a drain (or asecond terminal or the like) of a transistor can be distinguished fromeach other to specify the technical scope. Note that these expressionsare examples and it is not limited to the expressions. Here, X, Y, Z1,and Z2 each denote an object (e.g., a device, an element, a circuit, awiring, an electrode, a terminal, a conductive film, or a layer).

At least part of this embodiment can be implemented in combination withthe other embodiment described in this specification as appropriate.

Embodiment 5

The semiconductor device of one embodiment of the present invention canbe used for display devices, personal computers, or image reproducingdevices provided with recording media (typically, devices that reproducethe content of recording media such as DVDs: Digital Versatile Discs andhave displays capable of displaying the reproduced images). In addition,as electronic appliances that can employ the semiconductor device of oneembodiment of the present invention, cellular phones, game machinesincluding portable game machines, portable data terminals, e-bookreaders, cameras such as video cameras and digital still cameras,goggle-type displays (head mounted displays), navigation systems, audioreproducing devices (e.g., car audio systems and digital audio players),copiers, facsimiles, printers, multifunction printers, automated tellermachines (ATM), vending machines, and the like are given. FIGS. 30A to30F illustrate specific examples of these electronic appliances.

FIG. 30A is a portable game machine, which includes a housing 901, ahousing 902, a display portion 903, a display portion 904, a microphone905, a speaker 906, an operation key 907, a stylus 908, and the like.Note that although the portable game machine in FIG. 30A has the twodisplay portions 903 and 904, the number of display portions included ina portable game machine is not limited to this.

FIG. 30B is a portable data terminal, which includes a first housing911, a second housing 912, a first display portion 913, a second displayportion 914, a joint 915, an operation key 916, and the like. The firstdisplay portion 913 is provided in the first housing 911, and the seconddisplay portion 914 is provided in the second housing 912. The firsthousing 911 and the second housing 912 are connected to each other withthe joint 915, and the angle between the first housing 911 and thesecond housing 912 can be changed with the joint 915. Images displayedon the first display portion 913 may be switched in accordance with theangle at the joint 915 between the first housing 911 and the secondhousing 912. Furthermore, a display device with a position inputfunction may be used as at least one of the first display portion 913and the second display portion 914. Note that the position inputfunction can be added by providing a touch panel for a display device.Alternatively, the position input function can be added by provision ofa photoelectric conversion element called a photosensor for a pixelportion of a display device.

FIG. 30C is a notebook type personal computer, which includes a housing921, a display portion 922, a keyboard 923, a pointing device 924, andthe like.

FIG. 30D is an electric refrigerator-freezer, which includes a housing931, a refrigerator door 932, a freezer door 933, and the like.

FIG. 30E is a video camera, which includes a first housing 941, a secondhousing 942, a display portion 943, operation keys 944, a lens 945, ajoint 946, and the like. The operation keys 944 and the lens 945 areprovided in the first housing 941, and the display portion 943 isprovided in the second housing 942. Furthermore, the first housing 941and the second housing 942 are connected to each other with the joint946, and the angle between the first housing 941 and the second housing942 can be changed with the joint 946. Images displayed on the displayportion 943 may be switched in accordance with the angle at the joint946 between the first housing 941 and the second housing 942.

FIG. 30F is an ordinary vehicle, which includes a car body 951, wheels952, a dashboard 953, lights 954, and the like.

At least part of this embodiment can be implemented in combination withthe other embodiment described in this specification as appropriate.

Embodiment 6

In this embodiment, application examples of an RF tag of one embodimentof the present invention are described with reference to FIGS. 31A to31F. The RF tag is widely used and can be provided for, for example,products such as bills, coins, securities, bearer bonds, documents(driver's licenses, residence cards, or the like; see FIG. 31A),packaging containers (wrapping paper, bottles, or the like; see FIG.31C), recording media (DVDs, video tapes, or the like; see FIG. 31B),vehicles (bicycles or the like; see FIG. 31D), personal belongings(bags, glasses, or the like), foods, plants, animals, human bodies,clothing, household goods, medical supplies such as medicine orchemicals, or electronic appliances (e.g., liquid crystal displaydevices, EL display devices, television sets, or cellular phones), ortags on products (see FIG. 31E or FIG. 31F).

An RF tag 4000 of one embodiment of the present invention is fixed to aproduct by being attached to a surface thereof or embedded therein. Forexample, it is fixed to each product by being embedded in paper of abook, or embedded in an organic resin of a package. Since the RF tag4000 of one embodiment of the present invention can be reduced in size,thickness, and weight, it can be fixed to a product without spoiling thedesign of the product. Furthermore, bills, coins, securities, bearerbonds, documents, or the like can have an identification function bybeing provided with the RF tag 4000 of one embodiment of the presentinvention, and the identification function can be utilized to preventcounterfeiting. Moreover, the efficiency of a system such as aninspection system can be improved by providing the RF tag of oneembodiment of the present invention for packaging containers, recordingmedia, personal belongings, foods, clothing, household goods, electronicappliances, or the like. Furthermore, vehicles can also have highersecurity against theft or the like by being provided with the RF tag ofone embodiment of the present invention.

As described above, by using the RF tag of one embodiment of the presentinvention for each application described in this embodiment, power foroperation such as writing or reading of data can be reduced, whichresults in an increase in the maximum communication distance. Moreover,it can be preferably used for application in which data is notfrequently written or read, because data can be retained for anextremely long period even in the state where power is not supplied.

At least part of this embodiment can be implemented in combination withthe other embodiment described in this specification as appropriate.

Note that a content (or may be part of the content) described in oneembodiment may be applied to, combined with, or replaced by a differentcontent (or may be part of the different content) described in theembodiment and/or a content (or may be part of the content) described inone or a plurality of different embodiments.

Note that in each embodiment, a content described in the embodiment is acontent described with reference to a variety of diagrams or a contentdescribed with a text described in this specification.

Note that by combining a diagram (or may be part of the diagram)illustrated in one embodiment with another part of the diagram, adifferent diagram (or may be part of the different diagram) illustratedin the embodiment, and/or a diagram (or may be part of the diagram)illustrated in one or a plurality of different embodiments, much morediagrams can be formed.

Note that contents that are not specified in any drawing or text in thespecification can be excluded from one embodiment of the invention.Alternatively, when the range of a value that is defined by, forexample, the maximum and minimum values is described, the range isappropriately narrowed or part of the range is removed, whereby oneembodiment of the invention excluding part of the range can beconstituted. In this manner, it is possible to specify the technicalscope of one embodiment of the present invention so that a conventionaltechnology is excluded, for example.

As a specific example, a diagram of a circuit including a first to afifth transistors is illustrated. In that case, it can be specified thatthe circuit does not include a sixth transistor in the invention. It canbe specified that the circuit does not include a capacitor. It can bespecified that the circuit does not include a sixth transistor with aparticular connection structure in the invention. It can be specifiedthat the circuit does not include a capacitor with a particularconnection structure in the invention. For example, it can be specifiedthat a sixth transistor whose gate is connected to a gate of the thirdtransistor is not included in the invention. For example, it can bespecified that a capacitor whose first electrode is connected to thegate of the third transistor is not included in the invention.

As another specific example, a description of a value, “a voltage ispreferably higher than or equal to 3 V and lower than or equal to 10 V”is given. In that case, for example, it can be specified that the casewhere the voltage is higher than or equal to −2 V and lower than orequal to 1 V is excluded from one embodiment of the invention. Forexample, it can be specified that the case where the voltage is higherthan or equal to 13 V is excluded from one embodiment of the invention.Note that, for example, it can be specified that the voltage is higherthan or equal to 5 V and lower than or equal to 8 V in the invention.For example, it can be specified that the voltage is approximately 9 Vin the invention. For example, it can be specified that the voltage ishigher than or equal to 3 V and lower than or equal to 10 V but is not 9V in the invention. Note that even when the description “a value ispreferably in a certain range”, “a value preferably satisfies a certaincondition”, or the like is given, the value is not limited to thedescription. In other words, a description of a value that includes aterm “preferable”, “preferably”, or the like does not necessarily limitthe value.

As another specific example, a description “a voltage is preferred to be10 V” is given. In that case, for example, it can be specified that thecase where the voltage is higher than or equal to −2 V and lower than orequal to 1 V is excluded from one embodiment of the invention. Forexample, it can be specified that the case where the voltage is higherthan or equal to 13 V is excluded from one embodiment of the invention.

As another specific example, a description “a film is an insulatingfilm” is given to describe properties of a material. In that case, forexample, it can be specified that the case where the insulating film isan organic insulating film is excluded from one embodiment of theinvention. For example, it can be specified that the case where theinsulating film is an inorganic insulating film is excluded from oneembodiment of the invention. For example, it can be specified that thecase where the insulating film is a conductive film is excluded from oneembodiment of the invention. For example, it can be specified that thecase where the insulating film is a semiconductor film is excluded fromone embodiment of the invention.

As another specific example, the description of a stacked structure, “afilm is provided between an A film and a B film” is given. In that case,for example, it can be specified that the case where the film is astacked film of four or more layers is excluded from the invention. Forexample, it can be specified that the case where a conductive film isprovided between the A film and the film is excluded from the invention.

Note that various people can implement one embodiment of the inventiondescribed in this specification and the like. However, different peoplemay be involved in the implementation of the invention. For example, inthe case of a transmission/reception system, the following case ispossible: Company A manufactures and sells transmitting devices, andCompany B manufactures and sells receiving devices. As another example,in the case of a light-emitting device including a transistor and alight-emitting element, the following case is possible: Company Amanufactures and sells semiconductor devices in which transistors areformed, and Company B purchases the semiconductor devices, provideslight-emitting elements for the semiconductor devices, and completeslight-emitting devices.

In such a case, one embodiment of the invention can be constituted sothat a patent infringement can be claimed against each of Company A andCompany B. In other words, one embodiment of the invention can beconstituted so that only Company A implements the embodiment, andanother embodiment of the invention can be constituted so that onlyCompany B implements the embodiment. One embodiment of the inventionwith which a patent infringement suit can be filed against Company A orCompany B is clear and can be regarded as being disclosed in thisspecification or the like. For example, in the case of atransmission/reception system, even when this specification or the likedoes not include a description of the case where a transmitting deviceis used alone or the case where a receiving device is used alone, oneembodiment of the invention can be constituted by only the transmittingdevice and another embodiment of the invention can be constituted byonly the receiving device; those embodiments of the invention are clearand can be regarded as being disclosed in this specification or thelike. Another example is as follows: in the case of a light-emittingdevice including a transistor and a light-emitting element, even whenthis specification or the like does not include a description of thecase where a semiconductor device in which the transistor is formed isused alone or the case where a light-emitting device including thelight-emitting element is used alone, one embodiment of the inventioncan be constituted by only the semiconductor device in which thetransistor is formed and another embodiment of the invention can beconstituted by only the light-emitting device including thelight-emitting element; those embodiments of the invention are clear andcan be regarded as being disclosed in this specification or the like.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention can be clear even when portionsto be connected are not specified. Further, in the case where thecontent in which a portion to be connected is specified is disclosed inthis specification and the like, it can be determined that oneembodiment of the invention in which a portion to be connected is notspecified is disclosed in this specification and the like, in somecases. In particular, in the case where the number of portions to whichthe terminal is connected might be plural, it is not necessary tospecify the portions to which the terminal is connected. Therefore, itmight be possible to constitute one embodiment of the invention byspecifying only portions to which some of terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theportion to which a circuit is connected is specified. Alternatively, itmight be possible for those skilled in the art to specify the inventionwhen at least a function of a circuit is specified. In other words, whena function of a circuit is specified, one embodiment of the inventioncan be clear. Further, it can be determined that one embodiment of theinvention whose function is specified is disclosed in this specificationand the like. Therefore, when a portion to which a circuit is connectedis specified, the circuit is disclosed as one embodiment of theinvention even when a function is not specified, and one embodiment ofthe invention can be constituted. Alternatively, when a function of acircuit is specified, the circuit is disclosed as one embodiment of theinvention even when a portion to which a circuit is connected is notspecified, and one embodiment of the invention can be constituted.

Note that in this specification and the like, in a diagram or a textdescribed in one embodiment, it is possible to take out part of thediagram or the text and constitute one embodiment of the invention.Thus, in the case where a diagram or a text related to a certain portionis described, the context taken out from part of the diagram or the textis also disclosed as one embodiment of the invention, and one embodimentof the invention can be constituted. The embodiment of the invention isclear. Therefore, for example, in a diagram or text in which one or moreactive elements (e.g., transistors or diodes), wirings, passive elements(e.g., capacitors or resistors), conductive layers, insulating layers,semiconductor layers, organic materials, inorganic materials,components, devices, operating methods, manufacturing methods, or thelike are described, part of the diagram or the text is taken out, andone embodiment of the invention can be constituted. For example, from acircuit diagram in which N (N is an integer) circuit elements (e.g.,transistors or capacitors) are provided, it is possible to constituteone embodiment of the invention by taking out M (M is an integer, whereM<N) circuit elements (e.g., transistors or capacitors). As anotherexample, it is possible to constitute one embodiment of the invention bytaking out M (M is an integer, where M<N) layers from a cross-sectionalview in which N (N is an integer) layers are provided. As anotherexample, it is possible to constitute one embodiment of the invention bytaking out M (M is an integer, where M<N) elements from a flow chart inwhich N (N is an integer) elements are provided. For another example, itis possible to take out some given elements from a sentence “A includesB, C, D, E, or F” and constitute one embodiment of the invention, forexample, “A includes B and E”, “A includes E and F”, “A includes C, E,and F”, or “A includes B, C, D, and E”.

Note that in the case where at least one specific example is describedin a diagram or a text described in one embodiment in this specificationand the like, it will be readily appreciated by those skilled in the artthat a broader concept of the specific example can be derived.Therefore, in the diagram or the text described in one embodiment, inthe case where at least one specific example is described, a broaderconcept of the specific example is disclosed as one embodiment of theinvention, and one embodiment of the invention can be constituted. Theembodiment of the invention is clear.

Note that in this specification and the like, a content described in atleast a diagram (which may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, when a certain content is described in adiagram, the content is disclosed as one embodiment of the inventioneven when the content is not described with a text, and one embodimentof the invention can be constituted. In a similar manner, part of adiagram, which is taken out from the diagram, is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. The embodiment of the invention is clear.

REFERENCE SIGNS LIST

-   101 j: semiconductor layer,-   101 j_a: semiconductor layer,-   101 j_b: semiconductor layer,-   101 j_c: semiconductor layer,-   102 j: gate insulating film,-   103 j: gate electrode,-   104: conductive film,-   104 j_a: conductive layer,-   104 j_b: conductive layer,-   105 j: conductive layer,-   111: barrier film,-   112 j: insulating film,-   113 j: insulating film,-   114 j: insulating film,-   115 j: insulating film,-   116 j: insulating film,-   130 a: transistor,-   130 b: transistor,-   131: semiconductor substrate,-   132: semiconductor layer,-   133 a: low-resistance layer,-   133 b: low-resistance layer,-   134: gate insulating film,-   135: gate electrode,-   136: insulating film,-   137: insulating film,-   138: insulating film,-   141 j: plug,-   142 j: plug,-   143 j: plug,-   144 j: plug,-   145 j: plug,-   146 j: plug,-   147 j: plug,-   148 j: plug,-   151 j: conductive layer,-   152 j: conductive layer,-   153: conductive film,-   153 j: conductive layer,-   154 j: conductive layer,-   156 j: insulating film,-   171 j_a: low-resistance region,-   171 j_b: low-resistance region,-   176 a: region,-   176 b: region,-   190: transistor,-   198: transistor,-   199: transistor,-   201 j: semiconductor layer,-   201 j_a: semiconductor layer,-   201 j_b: semiconductor layer,-   201 j_c: semiconductor layer,-   202 j: gate insulating film,-   203 j: gate electrode,-   204 j: conductive film,-   204 j_a: conductive layer,-   204 j_b: conductive layer,-   205 j: conductive layer,-   212 j: insulating film,-   213 j: insulating film,-   214 j: insulating film,-   216 j: insulating film,-   230 a: transistor,-   230 b: transistor,-   232: semiconductor layer,-   233 a: low-resistance layer,-   233 b: low-resistance layer,-   235: gate electrode,-   300: memory cell array,-   500: peripheral circuit,-   700: semiconductor device,-   800: RF tag,-   801: communication device,-   802: antenna,-   803: radio signal,-   804: antenna,-   805: rectifier circuit,-   806: constant voltage circuit,-   807: demodulation circuit,-   808: modulation circuit,-   809: logic circuit,-   810: memory circuit,-   811: ROM,-   901: housing,-   902: housing,-   903: display portion,-   904: display portion,-   905: microphone,-   906: speaker,-   907: operation key,-   908: stylus,-   911: housing,-   912: housing,-   913: display portion,-   914: display portion,-   915: joint,-   916: operation key,-   921: housing,-   922: display portion,-   923: keyboard,-   924: pointing device,-   931: housing,-   932: refrigerator door,-   933: freezer door,-   941: housing,-   942: housing,-   943: display portion,-   944: operation key,-   945: lens,-   946: joint,-   951: car body,-   952: wheel,-   953: dashboard,-   954: light,-   1189: ROM interface,-   1190: substrate,-   1191: ALU,-   1192: ALU controller,-   1193: instruction decoder,-   1194: interrupt controller,-   1195: timing controller,-   1196: register,-   1197: register controller,-   1198: bus interface,-   1199: ROM,-   1200: memory element,-   1201: circuit,-   1202: circuit,-   1203: switch,-   1204: switch,-   1206: logic element,-   1207: capacitor,-   1208: capacitor,-   1209: transistor,-   1210: transistor,-   1213: transistor,-   1214: transistor,-   1220: circuit,-   2100: transistor,-   2200: transistor,-   4000: RF tag,-   5100: pellet,-   5120: substrate,-   5161: region.

This application is based on Japanese Patent Application serial no.2014-045406 filed with Japan Patent Office on Mar. 7, 2014, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: a memorycell including first to cth (c is a natural number of 2 or more) submemory cells each comprising a first transistor, a second transistor,and a capacitor, wherein: the first transistor overlaps with the secondtransistor with the capacitor interposed therebetween, a firstsemiconductor layer in the first transistor and a second semiconductorlayer in the second transistor include an oxide semiconductor, a firstterminal of the capacitor is electrically connected to one of source anddrain electrodes of the first transistor and a gate electrode of thesecond transistor, and jth (j is a natural number of 2 to c) sub memorycell is arranged over j−1th sub memory cell.
 2. A semiconductor devicecomprising: a memory cell including first to cth (c is a natural numberof 2 or more) sub memory cells each comprising a first transistor, asecond transistor, and a capacitor, wherein: the first transistoroverlaps with the second transistor with the capacitor interposedtherebetween, a first semiconductor layer in the first transistor and asecond semiconductor layer in the second transistor include an oxidesemiconductor, a first terminal of the capacitor is electricallyconnected to one of source and drain electrodes of the first transistorand a gate electrode of the second transistor, the second semiconductorlayer in jth (j is a natural number of 2 to c) sub memory cell and thefirst semiconductor layer in j−1th sub memory cell are in contact withan upper surface of a first insulating film, and the gate electrode ofthe second transistor in the jth sub memory cell and the gate electrodeof the first transistor in the j−1th sub memory cell are in contact witha lower surface of a second insulating film.
 3. A semiconductor devicecomprising: a memory cell including first to cth (c is a natural numberof 2 or more) sub memory cells each comprising a first transistor, asecond transistor, and a capacitor, wherein: jth (j is a natural numberof 2 to c) sub memory cell is arranged over j−1th sub memory cell, thefirst transistor is positioned over the second transistor with thecapacitor therebetween, a first semiconductor layer in the firsttransistor and a second semiconductor layer in the second transistorinclude an oxide semiconductor, a first terminal of the capacitor iselectrically connected to one of source and drain electrodes of thefirst transistor and a gate electrode of the second transistor, one ofthe first semiconductor layer and the second semiconductor layer in thefirst sub memory cell, and a third semiconductor layer in a thirdtransistor adjacent to the first sub memory cell are over and in contactwith a first insulating film, and one of the first semiconductor layerand the second semiconductor layer in the cth sub memory cell, and afourth semiconductor layer in a fourth transistor adjacent to the cthsub memory cell are over and in contact with a second insulating film.4. The semiconductor device according to claim 1, wherein: the oxidesemiconductor includes In, an element represented by M, and Zn, theatomic ratio of In to M and Zn of the oxide semiconductor included inthe first semiconductor layer satisfies In:M:Zn=g:h:i (each of g, h, iis a positive number), the atomic ratio of In to M and Zn of the oxidesemiconductor included in the second semiconductor layer satisfiesIn:M:Zn=d:e:f (each of d, e, f is a positive number), and g/(g+h+i) issmaller than d/(d+e+j).
 5. The semiconductor device according to claim2, wherein: the oxide semiconductor includes In, an element representedby M, and Zn, the atomic ratio of In to M and Zn of the oxidesemiconductor included in the first semiconductor layer satisfiesIn:M:Zn=g:h:i (each of g, h, i is a positive number), the atomic ratioof In to M and Zn of the oxide semiconductor included in the secondsemiconductor layer satisfies In:M:Zn=d:e:f (each of d, e, f is apositive number), and g/(g+h+i) is smaller than d/(d+e+j).
 6. Thesemiconductor device according to claim 3, wherein: the oxidesemiconductor includes In, an element represented by M, and Zn, theatomic ratio of In to M and Zn of the oxide semiconductor included inthe first semiconductor layer satisfies In:M:Zn=g:h:i (each of g, h, iis a positive number), the atomic ratio of In to M and Zn of the oxidesemiconductor included in the second semiconductor layer satisfiesIn:M:Zn=d:e:f (each of d, e, f is a positive number), and g/(g+h+i) issmaller than d/(d+e+j).
 7. The semiconductor device according to claim4, wherein the element represented by M is one of aluminum, gallium,yttrium, and tin.
 8. The semiconductor device according to claim 5,wherein the element represented by M is one of aluminum, gallium,yttrium, and tin.
 9. The semiconductor device according to claim 6,wherein the element represented by M is one of aluminum, gallium,yttrium, and tin.
 10. The semiconductor device according to claim 1,further comprising: a first wiring and a second wiring, wherein thefirst wiring is configured to electrically connect to the first terminalof the capacitor and the one of source and drain electrodes of the firsttransistor, and wherein the second is configured to electrically connectto the first terminal of the capacitor and the gate electrode of thesecond transistor.
 11. The semiconductor device according to claim 2,further comprising: a first wiring and a second wiring, wherein thefirst wiring is configured to electrically connect to the first terminalof the capacitor and the one of source and drain electrodes of the firsttransistor, and wherein the second is configured to electrically connectto the first terminal of the capacitor and the gate electrode of thesecond transistor.
 12. The semiconductor device according to claim 3,further comprising: a first wiring and a second wiring, wherein thefirst wiring is configured to electrically connect to the first terminalof the capacitor and the one of source and drain electrodes of the firsttransistor, and wherein the second is configured to electrically connectto the first terminal of the capacitor and the gate electrode of thesecond transistor.
 13. The semiconductor device according to claim 10,wherein the first wiring and the second wiring overlap with each other.14. The semiconductor device according to claim 11, wherein the firstwiring and the second wiring overlap with each other.
 15. Thesemiconductor device according to claim 12, wherein the first wiring andthe second wiring overlap with each other.
 16. The semiconductor deviceaccording to claim 1, wherein a mobility of the second transistor ishigher than that of the first transistor, and wherein an off-statecurrent of the first transistor is lower than that of the secondtransistor.
 17. The semiconductor device according to claim 2, wherein amobility of the second transistor is higher than that of the firsttransistor, and wherein an off-state current of the first transistor islower than that of the second transistor.
 18. The semiconductor deviceaccording to claim 3, wherein a mobility of the second transistor ishigher than that of the first transistor, and wherein an off-statecurrent of the first transistor is lower than that of the secondtransistor.